In a mixing bowl, combine the streusel ingredients—sugar, butter, and cocoa powder—using your fingers or a fork until they are moistened and crumbly.

In a small bowl, mix the espresso powder with boiling water, stirring until dissolved. Let this mixture cool to room temperature.

In another bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and any remaining cocoa powder.

In a large mixing bowl, whisk together the melted butter, granulated sugar, eggs, sour cream, and the cooled espresso mixture until smooth and creamy.

Gently fold the dry ingredients into the wet mixture until just combined.

Preheat your oven to 350°F (175°C) and grease a baking pan. Spoon half of the cake batter into the prepared pan and smooth it evenly. Sprinkle half of the reserved cocoa streusel over the batter.

Place the cake in the preheated oven and bake for 30-35 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.

Once baked, remove the cake from the oven and let it cool completely in the pan on a wire rack.