Adjust an oven rack to the middle position. Preheat the oven to 350F. Butter and flour two 8 by 2-inch square cake pans 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.

For the buttercream, put about 1 inch of water in a medium saucepan and bring it to a gentle boil. Melt the chocolate in a heatproof bowl set over the pan of boiling water, being careful not to let the water touch the bottom of the bowl. Stir constantly until just melted and set aside to cool slightly.

In the bowl of stand mixer fitted with a paddle, beat the butter on medium until light yellow and fluffy, about 3 minutes. Add the vanilla, corn syrup, and salt and beat on medium until combined. Turn the mixer to low and gradually add the confectioners' sugar. Beat at medium, stopping to scrape down the sides of the bowl as necessary, until smooth and creamy, 2-3 minutes. Add the chocolate and mix on low speed until no streaks remain.

Use half the buttercream on each cake, and frost as desired. Decorate with sprinkles if also desired.