How to Make Best Custard Donuts

INGREDIENTS:
Donut dough:
2 1/4 cups bread flour
1 large egg room temperature
1/2 cup + 2 tablespoons milk warm
3 tablespoons unsalted butter room temperature
5 tablespoons granulated sugar
1 teaspoon salt
1 envelope Instant Yeast 7 grams
Custard cream:
3 egg yolks room temperature
1 1/4 cup milk
1 1/2 tablespoons unsalted butter
4 tablespoons granulated sugar
4 tablespoons all-purpose flour
Pinch of salt
1 teaspoon vanilla extract