Preheat your oven to 350°F (175°C) and grease a standard loaf pan or bundt pan with butter.

In a medium saucepan, combine the butter, water, sugar, and cocoa powder. Heat over medium-low heat, stirring constantly until melted and smooth, then remove from heat.

Pour 1/2 cup of this chocolate liquid into a separate small bowl and set it aside to use as the glaze later.

Allow the remaining mixture in the saucepan to cool to lukewarm, then whisk in the eggs one at a time, followed by the vanilla extract.

Sift the flour and baking powder into the wet mixture and stir until just combined, ensuring no flour streaks remain.

Fold in the chopped walnuts, reserving a tablespoon for garnish if desired.

Pour the batter into the prepared pan and bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool for 10 minutes, then pour the reserved chocolate glaze over the warm cake before serving.