Heat the oven to 350 degrees F (175 degrees C), or 320 degrees F (160 degrees C) fan. Coat a 6-cup Bundt pan thoroughly with baking spray that contains flour.

Beat the butter and 1 cup sugar for 3 to 4 minutes, until pale and fluffy. Scrape the bowl as needed.

Beat in the eggs one at a time, then mix in the vanilla.

Whisk the flour, baking powder, and salt in a separate bowl. Add the dry mixture in three additions, alternating with the 100 ml milk. Mix on low just until combined.

Transfer about half of the batter to a clean bowl. Fold in the cocoa, 2 tablespoons sugar, and cinnamon. If the dark batter is thicker than the vanilla batter, fold in milk 1 teaspoon at a time until the textures are similar.

Spoon alternating mounds of both batters into the pan. Swirl once or twice with a long skewer, keeping it away from the greased surface.

Bake for 45 to 50 minutes, until a skewer inserted into the thickest part comes out clean or with a few moist crumbs.

Cool in the pan for 10 minutes, then invert onto a wire rack. Cool completely.

Melt the chocolate in 20-second bursts, stirring after each one. Drizzle over the cooled cake and let it set before slicing.