Preheat the oven to 350 F.

Prepare a 9×13 pan by lightly greasing it.

In a large mixing bowl, combine the starter, milk, and flour and whisk thoroughly.

In a separate bowl, beat together the sugar, oil, sour cream, vanilla, salt, baking soda, and cocoa.

The mixture will be grainy.

Add the eggs one at a time to this mixture, mixing well after each addition.

Gently combine the starter mixture into the chocolate mixture, whisking until smooth.

Pour the batter into the prepared pan.

Bake for 30-40 minutes, until it springs back when lightly pressed in the center, and a cake tendster in the center of the cake comes out clean.

Allow the cake to cool on a wire rack completely before frosting.

In the bowl of a stand mixer fitted with a whisk attachment, beat the butter until soft and fluffy.

Add in the confectioner’s sugar, cocoa powder, vanilla, and salt and beat until combined.

Add the milk, 1 tablespoon at a time, until you reach the desired consistency.