In a large bowl add 2 tablespoons of unsalted softened butter along with 1/4 cup granulated sugar, and 1/4 golden brown sugar. Mix it well using an electric hand mixer. Then add 1 large egg and 1/2 teaspoon of vanilla extract. Mix it again once more.
Now, in a separate bowl sift together these dry ingredients: 1 3/4 cups all-purpose flour, 1/4 cup + 1 tablespoon cocoa powder, 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 Tablespoon cornstarch, and 1/4 teaspoon salt, Now add 1/3 of the flour mixture to the butter and sugar mixture and begin working it in with a spatula.
Then add a third of your sour cream (so 1/3 cup at a time). Mix again.
Then add the second third of the flour mixture, mix again, then add another 1/3 cup of sour cream, mix.
Now add the last third of the flour, mix, then add the last 1/3 cup of sour cream.
Place the donut dough into a large sheet of plastic wrap and cover it tightly. Refrigerate for 1 hour or until firm.
Now lightly dust your work surface with flour and begin rolling out the chocolate donut dough to about 1/2 inch thick. 
Now use a donut cutter or 2 different sized biscuit cutters to cut out as many donuts as possible. It helps to dip the cutters in flour to prevent sticking. Now reshape the extra scraps, roll it out again to 1/2 inch thick, and cut as many donuts out once more. Repeat this step as needed. I got about 12 donuts.
Fill a heavy-bottomed pot with about 2 inches of corn oil and bring the oil temperature to 325 degrees Fahrenheit.
Carefully fry the donuts in batches for about 2 minutes per side. Keep a close eye on them to avoid overcooking them. Also, make sure to adjust the heat according to the thermometer. If it brings to climb too high, bring the heat down a notch.
Once the donuts are fried, place them on a plate lined with paper towels to absorb the extra oil.
Now, for the glaze, heat 1/2 cup of heavy cream to a light boil, then pour it over 1 cup of semisweet chocolate chips. Make sure all the chocolate is fully submerged, then use a whisk to mix it well. You may need to place it in the microwave for about 10 seconds at a time to reheat it. Once the glaze is smooth, whisk in 2 tablespoons of unsalted melted butter.
Now dip the slightly cooled donuts into the glaze and let them set on a cooling rack. Then enjoy with a cup of coffee or a glass of cold milk.