Preheat oven to 325 degrees. Line standard muffin tins with paper liners. Whisk together flour, baking powder, and salt.
With an electric mixer on medium-high speed, cream butter and sugar until pale and fluffy. 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 mixture in three batches, alternating with two additions of buttermilk and lemon juice, and beating until just combined after each.
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. Transfer tins to wire racks to cool completely before removing cupcakes. Cupcakes can be stored overnight at room temperature, or frozen up to 2 months, in airtight containers.
To finish, spread 1 tablespoon lemon curd onto middle of each cupcake. 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. 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.