Preheat oven to 325°F (165°C). Spray mini muffin pan generously with non-stick cooking spray or line with mini muffin liners.
In a large bowl, whisk together the flour, sugar, baking soda, and salt; set aside.
In a medium saucepan, combine the butter, water, and cocoa powder and heat over medium heat until boiling, stirring often. Pour into the dry mixture and mix just until combined.
Gently stir in the buttermilk and beaten eggs. The batter will be thin.
Scoop the batter into the prepared mini muffin pan, filling each well about 2/3 full.
Bake for 10-12 minutes, or until a toothpick inserted into the center comes out clean.
Cool the cake bites in the pan for 5 minutes, then remove and cool completely on a wire rack.
Frosting:
In a medium saucepan, combine the buttermilk, butter, and cocoa. Heat over medium heat until the mixture comes to a boil, whisking often.
Remove from the heat and whisk in the vanilla extract and powdered sugar until smooth.
Spoon the frosting over the cooled cake bites and allow a few minutes for the frosting to set.