Preheat your oven to 350°F (175°C). Grease your Bundt pan with melted butter, then dust it with cocoa powder to prevent sticking.
In a medium bowl, combine the finely chopped bittersweet chocolate, instant espresso powder, and boiling water. Stir until the chocolate is completely melted and smooth. Set aside to cool slightly.
In a large mixing bowl, combine the sour cream, melted chocolate mixture, and vanilla extract. Mix well until fully combined.
In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt. This helps ensure even distribution in the batter.
In another large bowl, beat the unsalted butter and light brown sugar together until light and fluffy, about 3-4 minutes.
Add the eggs to the butter-sugar mixture one at a time, mixing well after each addition to ensure they are fully incorporated.
Gradually add the dry ingredients to the wet mixture, alternating with the chocolate mixture. Start and end with the dry ingredients. Mix until just combined; be careful not to overmix.
Gently pour the batter into the prepared Bundt pan, smoothing the top with a spatula.
Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for about 15 minutes. Then, carefully invert the pan onto a cooling rack to let the cake cool completely.
While the cake cools, prepare the chocolate fudge icing. In a saucepan over medium heat, melt the unsalted butter. Once melted, whisk in the cocoa powder, evaporated milk, powdered sugar, vanilla extract, and salt until smooth and glossy.
Once the cake is completely cool, drizzle the chocolate fudge icing over the top, allowing it to cascade down the sides. Let it set for a few minutes before slicing.
