Prepare the cake mix as directed, stirring in lemon zest. Bake in two 7- or 8-inch round pans. Cool completely.
For the Frosting
Reduce the ½ cup lemon juice to 3 tablespoons by simmering it in a small saucepan and letting it evaporate. This will concentrate the flavor. Cool the reduction to room temperature.
Make Fluffy Vanilla Frosting but mix the lemon reduction into the butter before adding the confectioners sugar. Add the extra ¾ cup confectioners sugar at the end. Tint yellow, with food coloring, if desired. Use immediately or store airtight at room temperature for one day, chilled for a week.
Slice each cooled cake layer in half to make 4 cake layers. Place 1 layer on your cake plate or platter and top with ¼ cup lemon curd, spread evenly. Top with a spoonful of Lemon Frosting and spread over the curd; top with second cake layer. Spread that layer with lemon curd and frosting and repeat one more time with the third layer. Top with the last layer.
Frost the entire cake with Lemon Frosting. Sprinkle with decorative sprinkles or pipe on designs: With desired piping tip, decorate the top and bottom edges of the cake to finish it.
Cake can be stored at room temperature for a day and refrigerated up to a week.
My secret? A lemon juice reduction. Made in a matter of minutes, this fresh juice reduction is a super concentrated lemon bonanza, that is added to my frosting. The genius here is that because it's concentrated and reduced, it doesn't add a lot of liquid to my frosting. I still end up with fluffy frosting instead of a runny glaze, or frosting that loses it's flavor because you had to add too much confectioners' sugar.• Get lemon curd at the store in a jar and this cake comes together in no time!