Preheat the oven to 350°F. Heavily grease a mini muffin tin (or use nonstick spray).
In a large bowl, combine 2 cups sugar, 2 cups all purpose flour, 1/2 tsp salt, and 1 tsp baking soda. Set aside.
In a saucepan, combine 1 cup butter, 3/4 cup water, and 5 tbsp cocoa powder (Dutch-process recommended). Bring to a boil over medium heat, stirring until the butter melts and the cocoa is dissolved. Remove from heat.
Pour the hot butter–cocoa mixture over the dry ingredients. Use a hand mixer to mix until combined.
Add 1/2 cup buttermilk and 2 eggs to the batter. Mix with the hand mixer until smooth; the batter will be thin.
Place 1 tablespoon of batter into each cup of the greased mini muffin tin. Bake at 350°F for 10–12 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake bites cool in the pan for about 5 minutes, then gently remove them to a wire rack to cool completely.
For the frosting, in a medium saucepan heat 6 tbsp buttermilk (or milk), 1/2 cup butter, and 5 tbsp cocoa powder over medium heat, stirring, until the mixture is bubbly. Remove from heat.
Whisk in 1 lb powdered sugar and 1 tsp vanilla extract until the frosting is smooth and well combined. Stir in 1 cup chopped walnuts.
When the cake bites are completely cool, spoon about 1 tablespoon of frosting over each one. Let the frosting set before serving.