01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt until well combined.
03 - In a large bowl, beat butter and sugar together until light and fluffy, about 3 minutes. Add eggs one at a time, mixing well after each addition. Stir in lemon zest and vanilla extract.
04 - Combine milk and lemon juice in a small cup. Add dry ingredients to the butter mixture in three parts, alternating with the milk-lemon juice mixture, starting and ending with dry ingredients. Mix until just combined, being careful not to overmix.
05 - Divide batter evenly among cupcake liners, filling each about 3/4 full. Bake for 18–20 minutes, or until a toothpick inserted in the center comes out clean. Cool completely on a wire rack before filling.
06 - In a small saucepan over medium heat, whisk egg yolks, sugar, lemon juice, and zest constantly until thickened, about 5–7 minutes. Remove from heat and stir in butter until smooth and melted. Transfer to a bowl, cover with plastic wrap touching the surface, and cool completely.
07 - Once cupcakes and curd are completely cool, use a small knife or cupcake corer to remove the center of each cupcake, leaving about 1/2 inch at the bottom. Fill each cavity with about 1 teaspoon of cooled lemon curd.
08 - In a heatproof bowl set over a pot of simmering water, whisk egg whites and sugar constantly until sugar dissolves and mixture is warm to the touch, about 3 minutes. Remove from heat, add cream of tartar, and beat with an electric mixer until stiff, glossy peaks form, about 5 minutes. Beat in vanilla extract.
09 - Pipe or spoon meringue onto filled cupcakes, creating decorative swirls. Toast meringue using a kitchen torch until golden brown, or place cupcakes under the broiler for 1–2 minutes until lightly toasted, watching carefully to prevent burning.
10 - Serve immediately for best texture, or store in an airtight container in the refrigerator for up to 2 days. Bring to room temperature before serving.