Preheat the oven to 350°F (175°C). Lightly grease three to four donut hole pans, or two 24-cup mini muffin pans, ensuring the batter does not stick during baking.

In a large mixing bowl, whisk together the flour, unsweetened cocoa powder, baking powder, baking soda, and salt until well combined. Set aside.

In a separate bowl, whisk together the melted butter, granulated sugar, and brown sugar until the mixture is thick and smooth. Add the eggs, egg yolk, and vanilla extract. Mix thoroughly until fully incorporated.

In a measuring cup, mix the whole milk and sour cream until smooth. Alternately add the dry ingredients and the milk mixture into the wet ingredients, beginning and ending with the dry ingredients. Stir gently by hand using a spatula until the mixture is just combined and no flour remains visible.

Using a mini cookie scoop or spoon, fill each prepared pan cavity about ¾ full with batter. Bake in the preheated oven for approximately 10 minutes, or until a toothpick inserted into the center of a donut hole comes out clean.

Allow the baked donut holes to cool in the pans for several minutes before transferring them to a wire rack to cool completely.

In a medium bowl, whisk together the powdered sugar, 3 tablespoons of milk, and vanilla extract until smooth. If necessary, add additional milk, one teaspoon at a time, until the glaze reaches a pourable consistency.

Once the donut holes have cooled completely, dip each one into the glaze, allowing the excess to drip off. Return the glazed donut holes to the wire rack to set. For a more pronounced glaze coating, allow the first layer to dry slightly before dipping a second or third time as desired.

Once the glaze is fully set, the Glazed Chocolate Donut Holes are ready to enjoy.