Preheat the oven to 180C degrees.
Place melted butter and sugar in a stand mixer bowl fitted with a paddle attachment. Beat on medium speed until mixture is well combined, about 1 minute. Add in vanilla and beat until combined. Add and beat eggs one at time, mixing well after each addition. Once all eggs are added, beat mixture until it becomes light and fluffy.
In a separate medium size bowl add in 3 Tbsp of sugar, the cocoa powder, and water then whisk until mixture is smooth; set aside.
In a separate bowl mix the flour with the baking powder and salt
Gently stir in the milk in the butter and sugar mixture, alternately with the flour mixture, one-third at a time, ending with the flour mixture. Scrape the bowl once more, and gently beat for 1 minute.
Add half of the batter to the cocoa mixture and stir until blended.
Thoroughly grease a standard-size Bundt pan. Using an ice cream scoop or a tablespoon, pour one scoop of vanilla mixture into the pan. Now alternate and pour one scoop of chocolate mixture on top of the vanilla mixture. Continue to alternate between vanilla and chocolate layers until bundt pan is filled.
Bake it for 50 to 55min on 180C degrees or until a cake tester inserted into the center comes out clean. Cool cake in the pan on a wire rack for 15 minutes
While the cake cools prepare the glaze .Place chocolate and milk in a heatproof bowl. Place bowl over a saucepan of simmering water and allow chocolate to melt, stirring regularly.Remove the bowl from the heat and stir in the sugar. Mix until glaze is smooth.
Pour it over your bundt cake while the glaze is still warm.