Step 1: Preheat your oven to 350°F (175°C) to ensure even baking.
Step 2: Line a muffin tin with cupcake liners for easy cleanup and moisture retention.
Step 3: Melt the chopped bittersweet chocolate and unsalted butter together in a microwave-safe bowl. Microwave in 20-second intervals, stirring in between, until smooth. Set aside to cool slightly.
Step 4: In a medium mixing bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and fine sea salt until well combined.
Step 5: In another bowl, whisk together the granulated sugar, egg, vanilla extract, and sour cream until smooth. Stir in the melted chocolate mixture until well combined.
Step 6: Gradually add the dry ingredients to the wet mixture, alternating with the hot water. Mix until just combined. Add espresso powder if desired.
Step 7: Divide the batter evenly among the prepared muffin tin, filling each liner about 2/3 full. Bake for 18-20 minutes, or until a toothpick comes out clean.
Step 8: Allow the cupcakes to cool in the tin for about 5 minutes before transferring them to a cooling rack.
Step 9: Prepare the frosting by melting the chopped dark chocolate in the microwave. Beat the softened butter until creamy, then gradually add powdered sugar, vanilla extract, and a pinch of salt. Mix until combined. Add melted chocolate and milk; beat until fluffy.
Step 10: Once cooled, frost the cupcakes generously with the chocolate frosting. Drizzle with chocolate syrup if desired. Enjoy!
