For the Lemon Curd: Whisk sugar, eggs, egg yolks, and salt together in a medium heavy-bottomed saucepan. Add lemon zest and lemon juice and cook over medium heat, whisking constantly, until thickened and the consistency of pudding, 8 to 10 minutes.
Strain curd into medium bowl. Add butter and stir until completely incorporated. Place a piece of plastic wrap directly on the surface of curd and refrigerate until completely chilled, about 2 hours. Alternatively, place bowl in ice bath and stir constantly until custard is completely cooled, 10 to 15 minutes. Place plastic wrap directly on surface of curd and refrigerate until needed. (Curd may be prepared up to 2 days in advance).
For the Lemon Cupcakes: Place 24 cupcake liners in 2 (12-cup) baking tins. Lightly coat tins and liners with baking spray. Adjust oven racks to upper middle and lower middle positions and preheat oven to 350°F.
In food processor, pulse1 1/2 cups sugar and lemon zest until no zest strands remain.
Whisk all-purpose flour, cake flour, baking powder, and baking soda together in medium bowl; reserve.
Combine milk, lemon juice, and vanilla in glass measuring cup (mixture will curdle); reserve.
In large, clean, dry bowl, whip egg whites and salt with whisk attachment on medium speed until soft peaks form, about 2 minutes. Gradually add remaining 1/4 cup sugar and continue whipping until stiff peaks form, 1 to 2 minutes longer. (If you used the stand mixer bowl, transfer whites to separate bowl).
Beat butter and lemon sugar with paddle attachment on medium speed until light and fluffy, about 2 minutes. Add egg yolks, one at a time, beating well after each addition. Add flour mixture in three batches, alternating with milk mixture.
Stir in 1/3 of egg whites, then, fold in remaining egg whites with rubber spatula. Divide batter equally between prepared pans, filling each cup about 3/4 of the way.
Bake until tester inserted in center of cupcakes comes out clean, 20 to 25 minutes, rotating and alternating pans halfway through baking.
Transfer tins to cooling racks and cool 10 minutes in pans. Turn cupcakes out directly onto racks, then invert so the tops are facing up. Cool completely, about 30 minutes
For the Meringue Topping: Stir sugar, water, and corn syrup in heavy-bottomed medium saucepan over medium-low heat until sugar dissolves. Increase heat to medium-high and boil syrup until it registers 238°F on candy thermometer, 8 to 10 minutes.
While syrup boils, in large, clean, dry bowl, whip egg whites and salt with whisk attachment on medium speed until soft peaks form, about 2 minutes. With mixer running on medium-high speed, add syrup to egg whites in a slow, steady stream. Beat until outside of bowl is cool to the touch and whites are thick and glossy, about 7 minutes.
Transfer meringue to a pastry bag fitted with a 1/2-inch pastry tip. Alternatively, transfer meringue to a gallon-sized zipper-lock bag and cut off a 1/2-inch from the tip.
To Assemble: Once cooled, cut out circles in center of cupcakes, leaving about a 1/2-inch border around. The cutout will look like a cone. Lift the “cone” from the cupcake and trim off the pointy part to make a coin-like cap.
Fill each cupcake with curd and cover center with cap.
Pipe meringue onto cupcakes, smoothing out sides or making peaks decoratively. Brown meringues with torch. (If you don’t have a torch, heat broiler. Arrange frosted cupcakes on a baking sheet, 6 at a time and run under the broiler until browned, 10 to 30 seconds, depending on the broiler). Serve.
24 cupcake liners, 2 (12-cup) baking tins, food processor, electric mixer, rubber spatula, 2 cooling racks, pastry torch (optional), medium heavy-bottomed saucepan, whisk, strainer, candy thermometer, pastry bag and 1/2-inch pastry tip (or gallon-sized zipper-lock bag)
