Creme Brulee Donuts

Creme Brulee Donuts
Ingredients:
For the Donuts:
2 cups all-purpose flour
1/2 cup granulated sugar
2 teaspoons baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1/2 cup buttermilk
1/4 cup unsalted butter, melted
2 large eggs
1 teaspoon vanilla extract
For the Custard Filling:
1 cup whole milk
1/2 cup heavy cream
4 large egg yolks
1/4 cup granulated sugar
2 tablespoons cornstarch
1 teaspoon vanilla extract
For the Caramelized Sugar Topping:
Granulated sugar (for sprinkling)