Adjust an oven rack to the middle position. Preheat the oven to 350F. Butter and flour (or grease) a 10-inch Bundt pan.
Put the bittersweet chocolate in a small bowl. Pour the hot water over it and cover with a piece of plastic wrap. In a medium bowl or liquid measuring cup, whisk the sour cream, milk, canola oil, eggs, and vanilla.
In the bowl of a stand mixer fitted with a paddle, mix the flour, sugar, cocoa powder, ground cardamom, baking soda, baking powder, and salt on low until combined. (If the cocoa powder is lumpy, you can sift it into the other ingredients.) With the mixer running on low, slowly add the sour cream mixture. Increase the speed to medium and beat until combined, 20-30 seconds.
Whisk the chocolate and hot water together until completely smooth. With the mixer running on low, slowly pour the mixture into the batter and mix until just combined. Using a spatula, give the batter a couple of turns to make sure it is fully mixed.
Pour the batter evenly into the prepared pan. Bake 25-35 minutes, until a wooden skewer or toothpick comes out with a little bit of crumb.
Transfer the cake to a wire rack and let cool for 30 minutes. Turn the cake out onto the rack, and let cool completely. Once cool, the cake can be topped with chocolate ganache if desired.