In a bowl, sift together the cornstarch, flour, and baking powder. Set aside.
Beat the butter, sugar, brandy, and lemon zest together with an electric mixer on medium for 2 minutes, until the mixture turns a pale yellow and is light and fluffy. Add the egg yolks one at a time, beating on medium after each addition, until just combined. Add the lemon zest and beat on low until combined.