1. Preheat your oven to 350 F (175 C). Line a 12-cup muffin tin with paper liners.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, unsweetened cocoa powder, baking soda, and salt until well combined.

3. In a separate medium bowl, whisk together the active sourdough starter, milk, vegetable oil, vanilla extract, and the large egg until smooth and fully incorporated.

4. Pour the wet ingredient mixture into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Do not overmix.

5. Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. Bake for 12-15 minutes, or until a wooden skewer inserted into the center comes out clean.

6. While the cakes are cooling, combine the mini marshmallows and 2 tablespoons of milk in a microwave-safe bowl. Microwave on high for 30-60 seconds, stirring halfway through, until the marshmallows are melted and smooth. Stir in the powdered sugar until fully incorporated and smooth.

7. Once the mini cakes are completely cool, use a small knife or a round cookie cutter to carefully scoop out a small cavity from the center of each cake. Fill each cavity with a spoonful of the marshmallow filling.

8. If desired, melt the chocolate chips and butter or coconut oil in a microwave-safe bowl in 30-second intervals, stirring until smooth. Drizzle or spread the melted chocolate over the filled mini cakes. Let the glaze set before serving.