In the work bowl of a food processor, pulse strawberries until nearly smooth but a few small pieces remain. Transfer to a small saucepan; bring strawberry purée to a boil over medium-high heat. Reduce heat to medium-low, and cook, stirring frequently, until thickened and reduced to ? cup, 10 to 15 minutes. Remove from heat, and let cool completely.: When you pulse the strawberries , notice the fresh, green berry aroma that expands as the seeds break and juices release. Moving the purée to a saucepan, you will hear a soft, steady simmer and see the mixture go from a loose red liquid to a glossy, thicker texture. As it reduces, steam will carry a concentrated strawberry scent and the bubbling will slow; watch for a syrupy consistency that coats the back of a spoon. Reducing concentrates sugars, which is why this step is key for flavor intensity in the cake without making the batter too wet. A common mistake is to walk away and let the purée scorch, which produces a bitter note; stir frequently and keep the heat moderate to avoid burning. Let it cool completely so it does not loosen the batter when folded in.
Preheat oven to 350°F (180°C). Spray a 6-cup Nordic Ware Charlotte Cake Pan with baking spray with flour.: As the oven warms to the correct temperature, you will feel the kitchen heat up and may notice the slightly toasted scent that ovens give off. Preparing the pan ensures the cake releases cleanly; the flour in the spray creates a thin barrier that helps avoid sticking while still allowing even browning. Position the rack in the center of the oven to promote even rise. A frequent slip is not prepping the pan properly, causing the cake to cling and tear on unmolding; take a moment to coat the pan well, tapping out excess spray if needed.
In a medium bowl, whisk together flour, baking powder, salt, and baking soda.: Whisking these dry ingredients distributes the leavening evenly and aerates the flour slightly, promoting a more tender cake crumb. You will see the powder lighten in color and become uniform, with no visible lumps of baking powder or baking soda. This attention to blending prevents uneven pockets of rise, which could make the cake dome or collapse. Avoid overwhisking to the point of creating flour dust in the air; a few strokes to combine are sufficient.
In a large bowl, whisk together ? cup (133 grams) sugar, oil, and egg whites. Whisk in milk, lemon juice, and vanilla. Add flour mixture, and whisk until almost combined. Gently fold in strawberry purée and food coloring (if using) just until combined. Spread batter into prepared pan. Firmly tap pan on kitchen towel-lined counter to settle batter.: As you whisk the sugar with the oil and egg whites, you will notice the mixture become glossy and slightly thickened. The aroma will be faintly sweet with a hint of citrus from the lemon juice . Adding warmish room temperature liquids helps them incorporate smoothly; the batter should be homogenous but not overmixed. Folding in the reduced strawberry purée introduces fruit threads and color variations, and the batter should streak slightly pink before it blends. Spreading into the pan, you will see a silky surface; tapping helps trapped air escape and settles the batter into all the pan nooks. Overworking at this stage can develop gluten and toughen the cake, so stop folding as soon as visible streaks disappear.
Bake until cake springs back when lightly touched in center and a wooden pick inserted in center comes out clean, 20 to 25 minutes. Let cool in pan on a wire rack for 10 minutes. Invert cake onto a wire rack, and let cool completely.: While baking, the kitchen fills with a warm, slightly sweet smell that hints of cooked strawberries . The edges should pull back slightly from the pan and the top will turn a pale golden hue. The center will be set but springy; pressing gently should create a quick recovery rather than a deep dent. Inserting a wooden pick should return clean or with a few moist crumbs, not raw batter. Allowing the cake to rest in the pan prevents a sudden structural shock, and flipping onto a rack promotes even cooling and prevents sogginess. A common trap is removing the cake too early, which can cause the center to collapse; be patient and watch the spring test.
Spread Vanilla Bean Custard into center of cooled cake. Refrigerate until custard is set, about 1 hour.: When you spoon the custard into the cavity, its surface should be glossy and velvety, with a scent of warm vanilla. The custard should mound slightly but settle evenly, creating a smooth midsection that contrasts with the cake crumb. Chilling firms the custard and allows flavors to meld; the coldness also helps the subsequent brûlée sugar to caramelize cleanly without melting into the custard. A common mistake is adding the custard while the cake is still warm, which will cause the custard to thin and not set properly, so ensure the cake is completely cooled first.
Just before serving, sprinkle remaining 3 tablespoons (36 grams) sugar onto custard, leaving a 1/4-inch border around edges of custard. Using a handheld kitchen torch, carefully brown sugar. Serve immediately.: The final step sings with contrast; the sprinkled sugar should be evenly distributed and sit on the custard surface, ready to transform. When you torch the sugar, it will melt, bubble, and quickly darken to amber, emitting a light caramel scent. Aim for an even sweep of heat and watch closely because caramelized sugar can go from perfect to burnt in seconds. If you do not have a torch, a very hot broiler works but requires vigilance and immediate removal to avoid overheating. Serve immediately for the best textural contrast between crackling sugar and cool custard, as the sugar will slowly absorb moisture and lose crispness over time.
