1Preheat oven to 350F.
2Whisk the butter with the sugar in a large bowl until thick and pale in color. Add one by one the eggs, stirring at the same time until smooth. While stirring add flour one spoon at a time until mixture gets a thick texture.
3Grease the bottom and sides of 8.5” pan, sift a bit of flour, and tilt the pan so bottom and sides are evenly coated. Turn over; tap the bottom to remove any excess flour. Add the cake mixture and bake it for 35-40 minutes, until a skewer comes out clean.
4Remove from the oven and let it cool. Slice the cake into diamond shapes.
5In a saucepan over medium heat combine 1-1/2 cup sugar, 400gr water, 6 tablespoons chocolate powder and butter. Add vanilla seeds and 3 tablespoons dark rum. Simmer until the syrup gets homogeneous, stirring occasionally.
6While still hot pour the syrup and fill up the rims by pushing the cake pieces gently aside. Layer the top of the cake with the rest of the syrup.