01 - Preheat the oven to 350°F. Grease and flour a 9-inch round cake pan.
02 - In a large bowl, cream together the sugar and butter until pale and fluffy. Add the egg yolks one at a time, beating well after each addition. Mix in the lemon zest and juice.
03 - In a separate bowl, combine flour, baking powder, and salt. Add half the dry ingredients to the wet mixture, mix gently, then add the milk. Finish with the remaining dry ingredients and mix just until incorporated.
04 - In a clean bowl, beat the egg whites to stiff peaks. Gently fold the egg whites into the batter in three additions, taking care not to deflate the mixture.
05 - Pour the batter into the prepared pan and smooth the top. Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
06 - In a saucepan, warm the milk over medium heat until just simmering. In a separate bowl, whisk together sugar, egg yolks, and cornstarch until smooth. Slowly pour the hot milk into the egg mixture, whisking constantly. Return the mixture to the saucepan and cook over medium-low heat, stirring constantly, until thickened (5–7 minutes). Remove from heat, stir in butter, lemon zest, and lemon juice. Cover with plastic wrap directly on the surface and cool completely.
07 - Once both cake and custard are cool, slice the cake horizontally in half. Spread the lemon custard evenly over the bottom layer, then top with the second cake layer.
08 - Dust with powdered sugar and garnish with lemon slices or curls if desired. Chill for 30 minutes before serving for best texture.