1. Preheat your oven to 350 F (175 C). Grease and flour an 8×8 inch square baking pan or a 9-inch round cake pan. You can also line the bottom with parchment paper for easier removal.

2. In a medium bowl, whisk together the all-purpose flour, granulated sugar, baking soda, cocoa powder, and salt. Make sure there are no lumps and everything is evenly distributed.

3. In a large mixing bowl, using an electric mixer or a stand mixer with the paddle attachment, cream together the softened unsalted butter and the large egg until the mixture is light and fluffy (about 2-3 minutes).

4. In a separate small bowl, whisk together the sourdough starter (or discard), buttermilk, red food coloring, white vinegar, and vanilla extract until well combined.

5. Gradually add the dry ingredients to the butter-egg mixture, alternating with the sourdough-buttermilk mixture. Begin and end with the dry ingredients. Mix on low speed until just combined. Do not overmix. Scrape down the sides of the bowl as needed. The batter should be a vibrant red and relatively smooth.

6. Pour the batter evenly into your prepared baking pan. Place it in the preheated oven and bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. The cake should spring back lightly when gently touched.

7. Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for about 10-15 minutes. Then, carefully invert the cake onto the wire rack to cool completely.

8. While the cake cools, prepare your cream cheese frosting by beating softened cream cheese and butter until smooth, then gradually adding powdered sugar, vanilla, and milk until light and fluffy. Once the cake is completely cool, spread the frosting evenly over the top.

9. Slice your Sourdough Red Velvet Snack Cake and serve! It’s delicious on its own or with a cup of coffee or tea.