1. 1. Prepare Your Baking Pan: Preheat your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch square baking pan, or line it with parchment paper, leaving an overhang on two sides for easy removal.

2. 2. Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, granulated sugar, baking soda, unsweetened cocoa powder, and salt. Make sure there are no lumps and the ingredients are evenly distributed.

3. 3. Cream Wet Ingredients: In a large mixing bowl, using an electric mixer, cream together the softened unsalted butter and the large egg until light and fluffy, about 2-3 minutes.

4. 4. Add Liquids and Sourdough: Gradually beat in the buttermilk, unfed sourdough starter, vanilla extract, red food coloring, and white vinegar into the creamed butter mixture. Mix until just combined. The mixture might look a little curdled, but don’t worry, it will smooth out.

5. 5. Combine Wet and Dry: Slowly add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; overmixing can lead to a tough cake. Scrape down the sides of the bowl as needed to ensure everything is incorporated.

6. 6. Bake the Cake: Pour the batter evenly into your prepared 8×8 inch pan. Bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean.

7. 7. Cool the Cake: Remove the cake from the oven and let it cool in the pan on a wire rack for at least 15-20 minutes. Then, carefully invert it onto the wire rack to cool completely before frosting. This prevents the cake from becoming soggy.

8. 8. Prepare the Frosting: While the cake cools, make the sour cream frosting. In a medium bowl, beat the softened cream cheese and softened unsalted butter together until smooth and creamy. Gradually add the sifted powdered sugar, one cup at a time, mixing until fully incorporated. Beat in the sour cream and vanilla extract until the frosting is light and fluffy. If the frosting is too thick, add a tiny bit more sour cream; if too thin, add a bit more powdered sugar.

9. 9. Frost and Serve: Once the cake is completely cool, spread the sour cream frosting evenly over the top. Slice and serve!