For the Cake:

1. Preheat oven to 350°F (175°C). Grease a bundt pan generously with butter.
2. In a medium saucepan, combine butter, cacao powder, and water. Heat over medium until butter melts, stirring until smooth. Remove from heat.
3. In a large mixing bowl, whisk together flour, sugar, and baking soda. Gradually whisk in the warm chocolate mixture until smooth.
4. Add eggs, one at a time, whisking thoroughly.
5. Fold in sour cream, vanilla extract, and black garlic paste until the batter is uniform.
6. Pour into prepared bundt pan. Place in the oven and cook for about three-quarters of an hour, checking for doneness when a skewer pulls out without any batter residue.
7. Cool in pan for 15 minutes, then invert onto a wire rack to cool completely.

For the Sauce:

1. Add mango pieces, ginger preserves, sweetener, and a splash of water into a pot and mix over medium heat. Bring to a boil, then simmer for 15–20 minutes until thick and chunky.
2. Mash or blend for desired consistency.
3. Serve warm over slices of cake. Garnish with chocolate shavings.