Preparation

1. 1. Preheat oven to 350°F with the rack in the middle position. Coat a 9-inch square baking dish with softened butter or nonstick cooking spray; line with 2 pieces of parchment, leaving a 1- to 2-inch overhang on each side. Reserve.
2. 2. Prepare the crust. Put the flour, confectioners’ sugar, salt, and lemon zest in a medium mixing bowl. Mix on Speed 1 to fully combine, about 30 seconds. Add the cubed butter and mix on Speed 1 until the mixture becomes shaggy; this could take a little more than a minute (if necessary, increase to Speed 2). Transfer the crust mixture into the prepared pan and press onto the bottom and a little bit up the sides. Chill for 15 to 20 minutes to set. Once fully chilled, bake at 350°F for about 20 minutes, or until lightly browned. Remove and reserve. Reduce oven temperature to 300°F.
3. 3. Prepare the filling. Put the mascarpone in a small mixing bowl. Mix on Speeds 1 to 2 to make nice and creamy; reserve. Put theeggs into a medium mixing bowl (this can be the same one that was used for the crust) and mix on Speed 1 to break up and then add the sugar. Mix on Speeds 1 to 3 until light and thickened, about 1 minute. Add lemon juice, vanilla extract, and zest, and mix to incorporate. Add baking powder, salt, and flour, and mix to incorporate. Gently beat in the softened mascarpone. Pour on top of prepared crust. Bake at 300° for 25 to 30 minutes, or until just set.
4. 4. Dust with confectioners’ sugar before cutting and serving. Any lemon squares that are not consumed the day they’re made can be wrapped well and refrigerated for up to 5 days.