Preheat the oven 350°F/180°C and grease a 6 donut pan with oil. Set aside.
In a large bowl, whisk together vanilla, sour cream, egg, and butter.
2 tbsp butter, 1/2 cup sour cream, 1 tbsp vanilla, 1 egg
Add in the brown sugar and whisk.
1/3 cup light brown sugar
Add the flour, baking powder, and salt and fold in gently using a spatula until no dry streaks appear. Do not over-mix!
1 cup all-purpose flour, 1 tsp baking powder, 1/2 tsp salt
Place batter in a large plastic bag and cut off about ½" from the tip. Divide donut batter evenly between all 6 donuts.
Gently press diced peach pieces evenly into each donut. Then top off by sprinkling with streusel.
1/2 medium-sized peach
Bake for 18-20 minutes. You'll know it's ready when you push on the donuts and they spring back in the pan.
Cool for 2-3 minutes before gently running a knife around the donuts, popping them out of the pan, and placing on a cooling rack. Once slightly cooled, dust with icing sugar (optional).
Enjoy! I like to eat these donuts warm. They are best served day of!