To Make the Cake:

1. Add starter, milk, and flour to a large mixing bowl. Stir to combine. Cover with plastic wrap and let rest at room temperature for 2-3 hours. It should expand by a third in this time.
2. Preheat oven to 350°F. Lightly grease a 9-by-13-inch metal pan.
3. In the bowl of a stand mixer fitted with the paddle attachment, beat together sugar, oil, vanilla, salt, baking soda, and cocoa powder until combined.
4. Add eggs one at a time, beating well after each addition.
5. Add the wet chocolate mixture to the starter mixture, and stir until smooth. Pour batter into prepared pan.
6. Bake for 30-40 minutes, until a cake tester inserted into the center comes out clean.
7. Remove from oven. Set the pan on a cooling rack until completely cooled and ready to frost.

To Make the Frosting

1. Add butter and peanut butter to a large bowl. Use a hand mixer to beat until creamy and well-combined. 
2. Gradually add powdered sugar on low speed until combined. Scrape down the sides and bottom of the bowl as needed. 
3. Stir in vanilla extract, salt, and milk until well-combined. Beat on high speed until fluffy, about 30 seconds to 1 minute.
4. The mixture should be fluffy and creamy. If the frosting isn’t coming together and seems dry, add milk 1 teaspoon at a time, beating to incorporate after each addition. 
5. Frost cooled cake in the pan.