Preheat the oven to 325°F (165°C) and butter a 9x5 inch (23x12.5cm) loaf pan and line with parchment.

Place the cocoa powder and boiling water in a large mixing bowl and whisk until smooth. Let cool.

To the bowl add the butter, granulated sugar, brown sugar, and the vanilla extract and beat until light and fluffy (about 3-5 minutes).

Add the eggs, one at a time, and then beat in the flour until just combined.

Pour the batter into the prepared pan and bake for 70-80 minutes, or until a wooden skewer inserted into the center comes out clean.

Let cool in the pan for 10 minutes before lifting out of the pan and onto a wire rack to cool completely.

Once cooled, make the chocolate glaze: warm the chocolate, milk, and vanilla in the microwave or in a pot on the stove until melted and incorporated. Let cool for 5 minutes.

Drizzle the warm glaze over the cake and let it set before serving. Store in an airtight container at room temperature for up to 3 days or freeze for up to 2 months.