In a large bowl, bloom the cocoa powder by adding cocoa powder and hot coffee. Whisk until the mixture is combined and free of lumps. Let it rest for 5 to 10 minutes, or until it cools to room temperature.
To the cocoa-coffee mixture, add milk, oil, a large egg, sugar, sourdough starter and vanilla extract. Mix until well combined.
Add baking powder, salt and all-purpose flour, whisking until the batter is free of lumps.
Line a muffin pan with paper liners. Fill each liner about ¾ full with batter.
Don’t overfill the paper tins! Filling the tins right helps prevent overflow and gives your cupcakes a perfect dome.
Bake on the middle rack of a preheated 350°F (175°C) oven for 15 to 18 minutes, or until a toothpick inserted into the center comes out clean. Remove from the oven and let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
For the chocolate ganache, in a microwave-safe bowl, add the chocolate chips. Microwave in 20- to 30-second intervals, stirring between each, until fully melted and smooth. Add the heavy cream and whisk until the mixture is glossy and fully combined. For extra shine and a touch of sweetness, you can mix in 1 tablespoon of honey (optional). Let the ganache cool slightly until it thickens enough to spread.
Once both the cupcakes and ganache have cooled, top each cupcake with a generous spoonful of ganache. Use the back of a spoon or an offset spatula to spread it evenly.