Preheat Oven: Preheat oven to 180°C (350°F).
Prepare Buttermilk: Add vinegar to 1/2 cup milk. Let stand for 5 minutes until milk curdles and forms buttermilk.
Prepare Pan: Brush donut pan with melted butter.
Mix Ingredients: In a large bowl, combine flour, sugar, baking powder, salt, eggs, oil, vanilla extract, and prepared buttermilk. Mix until smooth.
Fill Pan: Pour batter into the donut pan, filling each cavity about 3/4 full.
Bake: Bake at 180°C (350°F) for 12-15 minutes or until donuts are light golden brown.
Cool Donuts: Allow donuts to rest for 5 minutes, then run a knife around the edges to unmold. Let cool completely on a wire rack.
Prepare Glaze: In a microwave-safe bowl, combine chocolate chips and 2 teaspoons milk. Microwave in 30-second intervals, stirring until melted and smooth. Allow to cool slightly.
Glaze Donuts: Dip the front of each donut into the chocolate glaze, then place on a cooling rack to drip and set.
Dust and Serve: Dust with cocoa powder and enjoy!