Preheat the oven to 375 degrees F. Line one 12-cup standard muffin tin with cupcake liners.
Sift together the flour, baking powder and 3/4 teaspoon of the salt.
Beat together the butter and 1 cup of the sugar with an electric mixer until pale and fluffy, about 5 minutes. Beat in the egg, yolk, zest and 1 1/2 teaspoons of the vanilla. Reduce the speed to low and add the flour mixture in three additions alternating with the milk, beginning and ending with the flour mixture. Divide the batter among the muffin cups and bake until a tester inserted in the middle of the cakes come out clean, 20 to 25 minutes. Cool the cakes for 10 minutes in the tin, and then remove and cool completely on a rack.
Put the lemon curd in a piping bag fitted with a 1/8-inch-round pastry tip. Insert the tip into the center of each cupcake and pipe in equal amounts of curd.
Combine the egg whites, corn syrup, the remaining 1/2 cup sugar, the remaining 1 teaspoon vanilla and the remaining pinch of salt in a metal bowl set over a saucepan of simmering water. Beat with an electric mixer until thick and fluffy, about 7 minutes. Remove from the heat and beat until the frosting is slightly cooled, about 2 minutes longer.
Mound the frosting on top of the cupcakes. If desired, use a kitchen torch to brown the frosting and garnish with a lemon candy.