Preheat oven to 350°F. Line a jelly roll (10×15”) pan with foil and spray with cooking spray. (Use the spray with flour or actually grease and flour the foil to avoid sticking.): When you start, your kitchen should smell neutral with a hint of citrus from the zest you prepared earlier, and the oven heat will radiate as you open it to place the pan. Lining the pan with foil and greasing it ensures the thin cake releases cleanly, which is crucial for the rolling step. If the pan is not well prepared, the cake may stick and tear when you flip it, which ruins the smooth spiral. A common mistake is skimping on greasing, so be generous with the spray or grease and flour the foil thoroughly.
Beat eggs at high speed for 2 minutes, until frothy and dark yellow. Beat in sugar, lemon juice, and zest.: As you beat, the sound will shift from liquid sloshing to a steady whipping, and the mixture will lighten to a deep yellow and become frothy. This aeration is the backbone of the sponge, trapping tiny air bubbles that expand in the oven. Adding the sugar and lemon ingredients at this stage helps dissolve sugar and distribute citrus oils throughout the batter. A troubleshooting point is overbeating after sugar is added which can deflate if you stop and restart vigorously; aim for even, steady mixing.
Whisk together salt, baking powder, and flour. Stir into wet ingredients just until blended.: The dry ingredients should look uniform and pale, with no clumps of flour or pockets of baking powder. Whisking aerates and evenly disperses the leavening so the cake rises uniformly. If your dry mix has lumps when folded, you risk streaks of dense batter in the finished cake. Resist the urge to sift aggressively into the wet mixture; a gentle whisk is usually enough.
Spread in prepared pan. Batter will be in a very thin layer and you will need to use a wooden spoon or spatula to spread it to all the corners of the pan. Bake for 9-11 minutes (mine took 10).: At this point you will note a change from glossy to matte as the dry ingredients hydrate, and the batter will become slightly thicker. Folding gently protects the air whipped into the eggs, preserving volume and producing a light texture. Over mixing here develops gluten and yields a tougher cake, so stop when you see no white streaks of flour . A common error is vigorous stirring which flattens the cake in the oven.
While the cake is baking, set a clean kitchen towel out on a large work surface. Sprinkle liberally with powdered sugar (about 1/4 cup). As soon as the cake comes out of the oven, turn it over on the kitchen towel sprinkled with powdered sugar. Remove foil carefully.: As you spread the batter, it should glide into corners, looking glossy and almost like a crepe batter. In the oven the top will set quickly and edges will pull away slightly from the foil, turning a light golden hue. Remove precisely when the center springs back; underbaking makes it hard to roll, overbaking dries it out and cracks when rolled. Avoid opening the oven repeatedly which can alter bake time.
Working at the short end, fold the edge of the towel over the cake. Roll tightly, rolling up the cake into the towel. Let cool completely while rolled, at least one hour.: The towel should feel dry and slightly grainy from the powdered sugar, which prevents sticking and adds a delicate outer sweetness. Turning the cake out while warm allows it to form the spiral shape without cracking. The foil comes off with a soft tearing sound; take care to peel gently to avoid pulling the cake. A common mistake is waiting too long to flip, which cools the cake and makes it less pliable.
While the cake is chilling, make the filling. Beat the heavy whipping cream and powdered sugar until stiff peaks form.: As you roll, you will hear a soft crepe like rustle from the towel and feel the cake give as it forms the spiral. Rolling while warm trains the cake to hold its shape and prevents cracks later. Letting it cool fully ensures the internal structure sets so the filling will not cause slumping. Avoid rolling loosely; an insufficiently tight roll results in gaps and a sloppy appearance.
Gently fold in the lemon curd. Chill until ready to use.: Cold heavy whipping cream whipped with powdered sugar changes from liquid gleam to thick peaks that hold shape, with the sound shifting to a denser whirr. Stiff peaks look glossy and stand tall on the whisk, which gives the filling body without heaviness. Overwhipping will turn the cream grainy and even into butter, so watch carefully and stop at the right moment.
When cake is cool, carefully unroll the towel. Spread the lemon whipped cream over the cake. (You might have some whipped cream left over.) Gently but tightly, re-roll the cake and wrap it in plastic wrap. Chill until it firms up a bit, at least one hour or overnight. Slice and serve. Cake will last covered in the refrigerator for up to 4 days.: The lemon curd ribbons through the whipped cream creating streaks of sunny color and a silky, tangy flavor. Folding should be gentle so the whipped cream retains volume, giving a light but flavorful filling. If you mix too vigorously the cream will deflate and the texture will become dense. A typical error is beating the curd in, which knocks out the air you worked to build.
Chill until ready to use.: The filling will feel cool and slightly firm after chilling, which makes spreading easier and prevents the roll from becoming squishy. Chilling also allows flavors to meld so each bite has a balanced profile. Do not skip chilling, or the filling may ooze when you slice the roll.
When cake is cool, carefully unroll the towel. Spread the lemon whipped cream over the cake. (You might have some whipped cream left over.) Gently but tightly, re-roll the cake and wrap it in plastic wrap.: Unrolling reveals a soft, slightly springy sponge with a sugared surface. Spreading should leave an even, thin layer so the roll remains neat and not overfilled. Re rolling too loosely yields air pockets, while rolling too tightly will force filling out. Wrap in plastic to keep moisture and shape, and chill so the filling sets.
Chill until it firms up a bit, at least one hour or overnight. Slice and serve. Cake will last covered in the refrigerator for up to 4 days.: After chilling, the roll will feel cool and compact, and the slices will present a clean spiral with a glossy filling. Serving cold highlights the creamy texture and keeps the spiral intact. Store tightly covered to prevent drying, and avoid leaving it at room temperature too long, which softens the cream and risks weeping.
