In a sauce pan or stock pot over medium-low heat, melt your butter and 2 oz. of unsweetened chocolate together.
To that, add your hot water and cup of sugar. Mix it all together, and set it aside to cool.
Sift your flour, baking soda, salt, and baking powder together.
Add your egg and vanilla extract to the chocolate mixture, and mix together.
Add your drop of vinegar to the milk to sour it slightly.
Start adding your milk and flour to the chocolate mixture, alternating a little bit of the flour, a little bit of the milk, until both are thoroughly mixed in.
Pour the mixture into a small round greased cake tin, and put in the oven for 30 - 35 minutes. Check it at 30 minutes - it might be done (you'll know that it is if you insert a cake tester (or a toothpick, or sharp knife) into the center and it comes out without anything sticking to it).
Allow the cake to cool, in the pan, for approximately 1 hour. After the hour is up, remove the cake from the pan onto a plate. The easiest way to do this is to invert a plate over the cake, and holding the plate and cake/pan in both hands, quickly flipping it (so the plate winds up on the bottom). Then with your knuckles, tap the bottom of the cake pan all around to loosen and free the cake from the pan. And then, carefully free the pan from the cake.
Put your heavy cream and sugar into a bowl, and whip them together for about 10 minutes to make your whipped cream.
Slice the cake into 2 halves, horizontally, so you have a top half and a bottom half. If you take a nice, long knife (or even a chef's knife) and insert it into the side, about half-way into the cake, and turn the plate (rather than rotating the knife around the plate) it's a lot less frustrating. Once you've cut the cake horizontally into 2 halves, remove the top piece and put on a separate plate to the side.
With your fingers, sprinkle your crème de cassis (or brandy or rum) over the top half and bottom half of your cake. Then, with a spatula, spread some of your whipped cream over the top of the bottom half. Next, place the top half of the cake back on, and spread whipped cream over the top of that. To garnish, scatter some raspberries and shards of unsweetened chocolate over the top, and place a sprig of fresh mint in the center.