This recipe takes tiramisu and swaps lemon for coffee while maintaining the rich, balanced flavor profile of a typical tiramisu. The lemon flavor is found in both the simple syrup and lemon curd, and the tart lemon is balanced with a rich mascarpone cream that is slightly sweetened. Since ladyfingers can be hard to find, you can bake up a French vanilla cake mix using egg whites instead of whole eggs to produce a spongy cake that is perfect for soaking up the lemon syrup.
In medium bowl, whisk the oil, 1 cup water, and egg whites until combined. Add the cake mix, and whisk until smooth. Pour the batter into a greased and parchment-lined rimmed sheet pan. Bake at 325°F for 15 to 18 minutes or until the cake gently springs back when pressed. Allow to cool completely. Cut into 2x3-inch rectangles.
In small saucepan, heat 1 cup water and granulated sugar over medium heat until the sugar is dissolved. Stir in lemon juice and zest. Allow to cool completely.
Meanwhile, in a stand mixer fitted with the paddle attachment, whisk mascarpone cheese, cream, powdered sugar, and vanilla on low until combined. Increase the speed to medium and whip until thick.
Spread a generous 1/2 cup of mascarpone cream in the bottom of a 13x9-inch (3-quart) glass baking dish. Dip both sides of half the cake pieces in the syrup, and place them over the cream. There can be a few gaps. Spread about 1 cup of cream over the cake. Spread the lemon curd over the cream in an even layer. Dip the remaining half of cake pieces in syrup, and place them over the curd. Spread a thin layer of cream (about 1/2 cup) over the cake. Transfer the remaining cream to a piping bag, and pipe dollops over the top. Finish the cake with additional lemon zest. Refrigerate for 1 hour. Cut into squares and serve.
