In a mixing bowl, combine sugar with egg yolk. Mix well.
Now add baking powder. Mix again.
Then, slowly add the flour into the bowl. Add the grated lemon zest. Set aside.
Use an electric mixer to beat the white eggs in another bowl. Then, add the cream of tartar. Mix for 10 minutes with sugar until fluffy.
Now, carefully mix the cake mixture into the bowl. Mix using a spatula.
Now, pour the cake mixture into the baking pan with custard mixture.
Put it inside the oven over a tray with water.
Bake the mixture for 45 minutes.
After 45 minutes, check it the cake using a toothpick and if it came out clean, it means it's cooked already.
Remove it from the oven and let it cool for 10 minutes.
After 10 minutes, turn the custard cake over a wide tray to get the custard cake from the baking pan.
Serve and enjoy!