In a large mixing bowl, cream together the softened unsalted butter and 2 cups of granulated sugar until light and fluffy. This process typically takes about 3-5 minutes.
Add the sour cream, eggs, and 2 teaspoons of vanilla extract into the mixture. Beat on medium speed until everything is well combined.
In a separate bowl, sift together the all-purpose flour, unsweetened cocoa powder, baking powder, salt, and instant espresso powder if you choose to use it. Gradually add this dry mixture to the wet ingredients, mixing just until combined.
Pour the batter into a greased loaf pan and smooth the top with a spatula. Bake in a preheated oven at 350°F (175°C) for about 60-70 minutes, or until a toothpick inserted into the center comes out clean.
While the cake is cooling, you can make the chocolate glaze. In a medium bowl, whisk together the powdered sugar, 6 tablespoons of cocoa powder, melted butter, 1-2 tablespoons of hot water, and ½ teaspoon of vanilla extract until smooth and glossy.
Once the pound cake is completely cooled, place it on a serving platter. Pour the chocolate glaze over the cake, allowing it to drip down the sides.