Preheat your oven to 350°F (175°C) and line your muffin tin with paper liners.
In a microwave-safe bowl or using a double boiler, melt the 1oz (28g) of bittersweet baking chocolate. Stir until smooth and set aside to cool slightly.
In a mixing bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and sea salt until well combined.
In another bowl, beat together the 2 tablespoons of unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.
Add the large egg and vanilla extract to the butter mixture, mixing until fully incorporated.
Stir in the sour cream and melted chocolate into the wet ingredients. Mix until smooth.
Gradually add the dry ingredients to the wet mixture, alternating with the hot water. Start and end with the dry ingredients, mixing until just combined. If using, stir in the espresso powder or instant coffee for an extra flavor boost.
Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
While the cupcakes cool, prepare the frosting. In a mixing bowl, beat together the softened 6 tablespoons of unsalted butter and 1 cup of powdered sugar until combined. Melt the 1oz (42g) of 70% cacao dark chocolate and mix it into the frosting, along with the vanilla extract, pinch of salt, and milk. Beat until creamy and smooth.
Once the cupcakes are completely cool, frost them generously with the chocolate frosting. If desired, drizzle with chocolate syrup for an extra indulgent touch.
