Preheat oven to 350°F (175°C).
Grease cake pan with butter thoroughly.
Mix flour, baking powder, and salt together.
Beat butter and sugar until light and fluffy.
Add eggs one at a time, beating well.
Stir in lemon juice and zest carefully.
Alternate adding flour mixture and milk slowly.
Pour batter into prepared cake pan evenly.
Bake for 25-30 minutes until golden brown.
Cool completely on wire rack before removing.
Heat milk in saucepan over medium heat.
Whisk egg yolks and sugar until pale.
Add cornstarch to egg mixture gradually.
Slowly pour hot milk into egg mixture.
Return mixture to saucepan, whisking constantly.
Cook until thick, about 3-5 minutes.
Remove from heat, add lemon juice.
Stir in butter and lemon zest.
Cool custard completely before using.
Cut cake horizontally into two layers.
Spread custard between layers and on top.