01 - Preheat oven to 350°F. Line a 12-cup muffin tin with cupcake liners.
02 - Whisk together flour, baking powder, baking soda, and salt in a bowl.
03 - Beat butter and sugar in a large bowl until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in lemon zest and juice.
04 - Add flour mixture in three parts, alternating with milk. Mix until just combined.
05 - Divide batter evenly among cupcake liners. Bake for 18–20 minutes until a toothpick inserted comes out clean. Cool completely on a wire rack.
06 - Whisk eggs, sugar, lemon juice, and zest in a heatproof bowl. Place over simmering water as a double boiler. Whisk constantly until thickened, about 8 minutes. Remove from heat and stir in butter until smooth. Cool completely.
07 - Remove the center of each cooled cupcake using a small knife or apple corer. Fill each cavity with cooled lemon curd.
08 - Beat egg whites and cream of tartar in a clean bowl until soft peaks form. Gradually add sugar while beating until stiff, glossy peaks form. Fold in vanilla.
09 - Pipe or spoon meringue onto filled cupcakes. Toast under a broiler for 1–2 minutes or use a kitchen torch until golden brown.
10 - Serve immediately or refrigerate for up to 2 days.