Preheat oven to 350 degrees F. Prepare mini muffin tins by lining each cup with mini paper liners or spraying cups liberally with baking spray that has flour in it.
Mix flour, sugar, baking soda, and salt in a large bowl. Set aside.
Combine water, cocoa powder, and butter in a medium saucepan. Bring to a boil, whisking often. Remove from heat, pour into the bowl with dry ingredients. Mix together until just combined.
Add buttermilk and eggs and mix to combine. The batter will be thin.
Using a 1 tablespoon cookie scoop, place one scoop of batter in each muffin cup.
Bake for 10-12 minutes or until cakes bounce back when lightly touched. Remove from oven and transfer bites to a rack to cool.If liners were not used, let bites cool for about 10 minutes in the pan. Then using the tip of a knife, gently remove from the pan and place on a wire rack to cool completely.
In a medium saucepan, heat over medium heat buttermilk, butter and cocoa until bubbly. Mix in sugar and vanilla until smooth. Add in nuts and blend until incorporated.
Carefully spoon about a tablespoon of frosting over each cake bite and let frosting set. Some frosting may run over the sides, that's okay. Store in an air-tight container.
If you don't have buttermilk on hand, you can make sour milk instead. Add 1 teaspoon white vinegar to 1 cup milk and let it sit for a few minutes. Add 6 tablespoons to recipe as directed.