Preheat oven, prep muffin pan, and whisk dry ingredients: Preheat oven to 325°F. Line standard muffin tins with paper liners. Whisk together flour, baking powder, and salt.

Cream butter and sugar: With an electric mixer on medium-high speed, cream butter and sugar until pale and fluffy.

Add eggs, one at a time: Add eggs, one at a time, beating until each is until incorporated, scraping down sides of bowl as needed. Beat in zest and vanilla.

Add flour in batches, alternating with buttermilk: Add flour mixture in three batches, alternating with two additions of buttermilk and lemon juice, and beating until just combined after each.

Divide batter and bake: Divide batter evenly among lined cups, filling each three-quarters full. Bake, rotating tins halfway through, until golden brown and a cake tester inserted in centers comes out clean, about 25 minutes.

Cool: Transfer tins to wire racks to cool completely before removing cupcakes.

Top with lemon curd: To finish, spread 1 tablespoon lemon curd onto middle of each cupcake.

Pipe frosting: Fill a pastry bag fitted with a large open-star tip (Ateco #828 or Wilton #8B) with frosting. Pipe frosting onto each cupcake, swirling tip slightly and releasing as you pull up to form a peak.

Torch frosting to brown: Hold a small kitchen torch 3 to 4 inches from surface of frosting, and wave it back and forth until frosting is lightly browned all over. Serve immediately.