In a medium-sized bowl, add 1 cup all-purpose flour, ½ cup light brown sugar, packed, ½ teaspoon salt, 1 teaspoon baking powder. Use a wire whisk to blend together.
Add to the dry mixture, ½ cup full-fat sour cream, 2½ Tablespoons melted butter, 1 large egg, and 3 tablespoons of buttermilk.
Carefully mix ingredients together being careful not to over mix.
Place donut mixture into a large plastic zip lock bag. Snip off bottom corner and pipe dough into the prepared donut baking sheet.
Top each donut with 1½ Tablespoons of crumb topping.
Bake at 350° for 11–15 minutes or until lightly brown.
If dough has covered the hole in the baking pan take a butter knife and carefully push the dough back towards the donut while in the baking pan.
Allow donuts to cool in baking pan for 10 minutes.
Remove donuts from baking pan and place on a cooling rack. Allow to cool until at least lukewarm.
Drizzle with Vanilla Icing.