Preheat oven to 350°F and grease a mini muffin tin.
Mix dry ingredients: cake mix, flour, cocoa, and salt in a bowl.
Whisk coffee, oil, sour cream, eggs, and vanilla in a separate bowl.
Combine wet and dry mixtures to make a smooth batter.
Spoon batter into muffin wells ¾ full. Bake for 10–12 minutes.
Make glaze: melt butter, stir in milk and cocoa, whisk in sugar and vanilla.
Spoon glaze over warm bites and allow to cool before serving.
Notes
Use a squeeze bottle for clean glazing. Add crushed pecans or mini chocolate chips for variety. Bites freeze well and stay fresh in a container for up to 4 days.