• Preheat the oven to 350°F (180°C). Line a 6-cup muffin pan with cupcake liners, then set it aside.

Make the Cupcakes:

• Whisk together the flour, cocoa powder, baking soda, and salt, in a small bowl, then set aside.
• Place the chocolate and butter in a large heat-safe bowl. Melt the chocolate and butter in the microwave, heating in 30-second intervals and mixing thoroughly between each interval. Once the chocolate is fully melted, remove the bowl from the heat, and stir in the sugar.
• Add the egg, vanilla, and sour cream, and whisk until the mixture is well combined, scraping down the sides and up the bottom of the bowl as needed.
• Add in the dry ingredients and whisk until they are just incorporated.
• In a small cup, whisk together the hot water and espresso powder (if using), then add that mixture into the batter and whisk until completely combined and smooth, about 1 minute. (Do this carefully as the water will splash around at first!)
• Spoon the batter into the prepared pan, filling each liner until almost full; you’ll use about 4 tablespoons (62 g) of batter per cup. Bake the cupcakes for 18 to 20 minutes, or until a toothpick inserted into the center comes out relatively clean with a few moist crumbs. Allow the cupcakes to cool completely before frosting them.

Make the frosting:

• Begin by melting the chocolate. Place the chopped chocolate in a heat-safe bowl and melt the chocolate in the microwave, heating it in 30-second intervals and mixing thoroughly between each interval. Once the chocolate is fully melted, remove the bowl from the heat and set it aside to cool slightly.
• In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a handheld mixer, beat the butter on medium speed until creamy, about 1 minute. Add the sugar, vanilla, milk, and salt. Beat on low speed for 30 seconds, then increase speed to medium-high, and beat for 2 full minutes.
• Add the melted chocolate, and beat the mixture on high speed for another 1 to 2 minutes, until the frosting is fluffy and the ingredients are completely combined. Add more powdered sugar if the frosting is too thin, more milk if the frosting is too thick, or an extra pinch of salt if the frosting is too sweet. The frosting should be soft and creamy, but not runny.
• Frost the cooled cupcakes however you’d like and garnish them with the chocolate syrup, if using. I used a large open star piping tip (Wilton 1M) for the cupcakes pictured.
• Store leftovers in an airtight container in the refrigerator for up to 4 days. Place the cupcakes on the counter about an hour before you plan to serve them, so they can come to room temperature.