• Preheat oven to 400F and spray 12 wells of a donut pan with nonstick spray. Set aside.
• In the bowl of a stand mixer fitted with the paddle attachment (or in a large mixing bowl with a handheld electric mixer), combine Bisquick baking mix, sugar, sour cream, milk, egg, oil, and vanilla extract. Mix on low speed to incorporate, then scrape down the sides of the bowl and mix on medium speed until just smooth.
• Spoon the batter into the prepared wells of the donut pan. Bake in preheated oven for 9-12 minutes, or until a toothpick inserted in the center of a donut comes out clean.
• Remove from oven and let the donuts cool in the pan for about 5 minutes, then invert the donuts onto a wire rack (so the smoothest side is facing up) and proceed as directed below if you are adding a topping to the donuts.

If Using Cinnamon Sugar Topping:

• Add melted butter to a shallow dish or bowl. In a separate shallow dish or bowl, combine sugar and cinnamon together.
• While the donuts are still warm, dip the smooth top surface of a donut into the melted butter. Then dip the same surface into the cinnamon sugar mixture and return to the wire rack. Repeat with remaining donuts. Enjoy warm or allow to cool completely on the wire rack.

If Using Chocolate Glaze Topping:

• Once the donuts are completely cooled, combine chocolate chips, butter, and milk in a small microwave-safe bowl. Microwave for 30 seconds, then stir well. If needed, microwave for an additional 15 seconds to melt completely.
• Add sifted powdered sugar to the melted chocolate mixture, then whisk well until fully combined and smooth.
• Dip the smooth top surface of each donut into the chocolate glaze, then return the donuts to the wire rack (with the chocolate side up). If desired, add sprinkles to the wet chocolate glaze. Let the donuts rest for about 15 minutes, which will allow the glaze to harden to a satiny finish.