Preheat oven & prep pan:
Preheat your oven to 325°F (165°C). Grease and line a 9×5-inch loaf pan with parchment paper.

Make cocoa mixture:
In a medium bowl, whisk together the unsweetened cocoa powder and boiling water until smooth. Let cool to room temperature.

Cream butter & sugars:
In a large mixing bowl, cream the softened butter, granulated sugar, and dark brown sugar until light and fluffy, about 3–4 minutes.

Add vanilla & eggs:
Beat in the vanilla extract, then add the eggs one at a time, mixing well after each addition.

Combine:
Stir in the cooled cocoa mixture. Gradually add the flour and mix until just combined—do not overmix.

Bake:
Pour batter into the prepared loaf pan and smooth the top. Bake for 60–70 minutes, or until a toothpick inserted into the center comes out mostly clean.

Cool:
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Make glaze:
In a small saucepan over low heat, heat milk until warm (not boiling), then pour it over the chopped chocolate in a bowl. Let sit for 1 minute, then stir until smooth. Stir in vanilla extract.

Glaze & serve:
Once the cake is fully cooled, pour the chocolate glaze over the top. Let set slightly before slicing.