Preheat oven to 400 degrees F.: You will notice the oven beginning to radiate steady heat, and that initial burst at a higher temperature helps the muffins get immediate oven spring. A warm oven means the leavening gases expand quickly, producing taller tops. Common mistake to avoid is not preheating long enough, which can cause uneven rise and dense centers.
Line a muffin pan with paper liners or grease lightly the muffin cups. Set aside.: As you prepare the pan you can hear the faint scrape of a buttered brush or feel the slickness of the liners, both creating a release surface so muffins lift out cleanly. Proper greasing prevents sticking and preserves tops that look attractive. A frequent error is over greasing, which can lead to flat bottoms and extra oiliness.
In a large mixing bowl, whisk together the dry ingredients (flour, baking powder, baking soda, salt and sugar). Whisk in the lemon zest. Add raspberries (saving 12 of them for the top of each muffin - if desired). Set aside.: As you whisk the dry mix it should look uniform, with the baking powder and soda evenly dotted throughout, and the zest will release aromatic oils that scent the air. Gently folding in the raspberries at this stage ensures they are distributed without crushing, preserving intact fruit pieces. Avoid vigorously stirring at this point, crushing the raspberries, which can turn the batter uniformly pink and make the muffins denser.
In a medium mixing bowl, whisk together buttermilk/sour milk, vanilla, egg and oil.: The wet mixture should look smooth and slightly glossy, with the egg fully incorporated into the liquid. Whisking aerates it just enough to help with lift, and the buttermilk creates tangy steam as it heats. A common mistake is adding cold eggs straight from the fridge, which can make the batter seize or mix unevenly, so let them sit briefly if you can.
Add the wet ingredients to the dry ingredients and stir in with a wooden spoon until well combined. Do not overmix. Slightly lumpy batter is okay.: As you fold, watch the batter transition from dry flour patches to a cohesive, slightly lumpy mass, with streaks of cream and flecks of raspberry. You should stop when most flour is just incorporated to avoid developing gluten, which causes tough muffins. One pitfall is overmixing until the batter is uniform and glossy, that will lead to tight crumb and dense muffins.
Scoop the batter with an ice cream scoop onto the prepared muffin pan. Place reserved berries on top of each muffin.: Using a scoop gives even portions that bake at the same rate, and the batter should mound slightly above the rim for a domed top. Pressing the reserved raspberry on top creates a pretty finish and a burst of fruit at the surface. Avoid packing the batter into the cups, which can compress the structure and hinder rise.
Bake for 5 minutes at 400 degrees. Without opening the oven, lower the temperature to 375 degrees and bake for 14 to 17 more minutes, or until the toothpick inserted into each muffin comes out clean and the tops are golden.: The initial high heat jumpstarts oven spring, producing tender domes, while lowering the temperature prevents overbrowning and ensures the centers finish cooking gently. During baking you might hear a faint settling and see the crust take on a soft golden hue that darkens slightly, which is a good cue. A common mistake is repeatedly opening the oven to check, letting heat escape and collapsing the tops, so resist peeking.
Cool muffins completely before serving.: As the muffins cool you'll notice steam release and the crumb set into a tender texture. Cooling stops carryover cooking and helps the crumb stabilize, so cutting too soon can leave you with gummy interiors. The usual error is diving in while still hot, which can make them fall apart and mask their true texture.
