Adjust an oven rack to the middle position. Preheat the oven to 350F. Butter and flour two 8 by 2-inch square cake pans (8 or 9-inch circular pans can also be used) and line the bottoms with parchment paper.
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, baking soda, baking powder, and salt on low until combined. 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 chocolate 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 pans. Bake 25-35 minutes, until a wooden skewer or toothpick comes out with the tiniest bit of crumb.
Transfer the cakes to a wire rack and let cool for 30 minutes. Turn the cakes out onto a half sheet pan lined with parchment paper, remove the parchment paper from the bottom of the cakes, and let the cakes cool completely upside-down (this will help deflate any doming). Once cool, the cakes can be wrapped in plastic and refrigerated overnight or frosted (cakes can also be frozen unfrosted for up to one month).