Preheat your oven to 325°F. Grease a 24-cup mini muffin pan.
In a microwave-safe bowl, melt the butter. Add the cocoa, and stir until combined well.
Bring 1 cup of water to a boil in the microwave or on the stovetop. Pour the boiling water into the butter cocoa mixture and stir for 30 seconds.
In a small bowl, whisk together buttermilk, eggs, and vanilla extract.
In a large mixing bowl, combine flour, sugar, baking soda, and a pinch of salt.
Pour the cocoa mixture into the dry ingredients, and mix until combined.
Lastly, stir the buttermilk mixture into the batter.
Scoop the batter into the prepared mini-muffin pan using a 1 tablespoon scoop. Bake 14-16 minutes or until a toothpick inserted comes out clean.
Allow the cake bites to cool 10 minutes before transferring to a cooling rack.