Preheat your oven to 350°F (175°C). Grease your Bundt pan generously with unsalted butter and dust it with flour, tapping out the excess.
In a large mixing bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set this mixture aside.
In a medium saucepan, bring the water to a boil. Once boiling, remove it from the heat and set it aside to cool slightly.
In a separate bowl, beat the unsalted butter and granulated sugar together until light and fluffy, about 3-5 minutes using an electric mixer.
Add the eggs one at a time to the butter mixture, mixing well after each addition. Then add the sour cream (or Greek yogurt) and vanilla extract, stirring until fully combined.
Gradually add the dry ingredients to the wet ingredients, alternating with the warm water. Start and end with the dry ingredients, mixing until just combined. Be careful not to overmix!
Pour the batter into the prepared Bundt pan, smoothing the top with a spatula.
Bake in your preheated oven for 50-60 minutes, or until a toothpick inserted in the center comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for about 10-15 minutes. Then carefully invert it onto a cooling rack to cool completely.
While the cake is cooling, prepare the chocolate ganache. In a double boiler or microwave, melt the bittersweet chocolate with the heavy cream and granulated sugar. Stir until smooth and glossy, then add the corn syrup for a glossy finish.
Once the cake is completely cool, pour the ganache over the top, allowing it to drip down the sides for that beautiful Bundt cake presentation.