Preheat your oven to 350°F . Grease and line an 8-inch round cake pan with parchment paper.
In a medium bowl, combine the hot espresso shots with the cocoa powder and whisk. This will allow the cocoa powder to bloom and have a stronger flavor. Add in the sourdough starter and milk. Whisk to combine.
1/2 cup sourdough starter discard, 1/4 cup unsweetened cocoa powder, 2 espresso shots, 1/3 cup whole milk (or plant-based milk)
Add the chopped chocolate and the oil in a large mixing bowl and melt in the microwave in 30-second increments, stirring in between, until thoroughly melted. Add in the sugar and brown sugar and whisk to combine.
1/2 cups granulated sugar, 1/3 cup vegetable oil, 3 oz semi-sweet chocolate bar, 1/4 cup light brown sugar
Whisk in the egg, vanilla, and the bloomed cocoa powder and sourdough mixture.
1 large egg, 1 tsp vanilla extract
Sift the flour, baking soda, baking powder, and salt. , Mix until just combined.
3/4 cups all-purpose flour, 1/2 tsp baking soda, 1/2 tsp baking powder, 1/4 tsp salt
Pour the batter into the prepared pan. Bake for 28-35 minutes, or until a toothpick inserted in the center comes out clean.
Cool the cake in the pan for 10 minutes, then transfer to a wire rack to cool completely.