Preheat the oven to 356 °F (180 °C).

In a bowl, beat the butter and sugar until creamy. Add the egg and continue beating until well combined. Finally, incorporate the flour and baking powder until you obtain an easy-to-work dough.

Press the dough with your fingers into the bottom and sides of a removable bottom cake pan. Prick the dough with a fork to prevent it from puffing up.

Bake for 15 to 20 minutes or until the crust is golden brown. Remove from the oven and let it cool.

In a bowl, dissolve the cornstarch in 1 cup of milk. Whisk until smooth. Add the egg yolks and continue whisking until everything is well combined. Set aside.

In a small saucepan, heat the remaining 2 cups of milk, sugar, and poppy seeds over medium heat, stirring occasionally, until it starts to boil.

Gradually pour the cornstarch mixture into the saucepan, whisking constantly. Once it starts boiling again, cook for an additional 2 minutes, stirring constantly.

Pour the mixture over the cooled crust and let it cool to room temperature before refrigerating.