For the dough,

Preheat oven to 180C or 350F.
Butter and flour a tart pan with a removable bottom.
In a large bowl, work the butter until soft with a fork, add sugar, and mix well for about 3 minutes. Add the egg and mix well. Add the flour with the baking powder, initially mixing with a fork, but finish kneading gently by hand.
Form a disk and move to the prepared tart pan. Press the dough to coat the bottom and edges of the tart pan using your fingers or the palm of your hand.
Bake for 16 to 18 minutes until it is golden. Remove from oven and let cool on a rack without unmolding.

For the custard,

Place poppy seeds, sugar, and 2 cups of milk in a medium saucepan. Cook over medium heat, occasionally stirring until boiling.
Mix the cornstarch, egg yolks, and one cup of milk. Whisk with a balloon whisk for 2 minutes. You must ensure that all the cornstarch is dissolved to smooth the cream.
Once the milk with the poppies boils, add it as a thin thread to the yolk mixture, whisking all the time. Return everything to the pot and continue cooking over medium heat until thickened, about 5 minutes, stirring. Once it boils, cook for 1 minute more. Remove from heat and add butter, mixing until fully incorporated.
Pour over the baked crust, don’t worry if it’s still hot. Spread flat.

Let it cool and enjoy it.