Butter an 8-inch springform pan.Cut a circle of wax paper to fit the bottom of the pan and place inside. Preheat oven to 300˚F.
Preparing the cake batter
Place ¾ cup + 2 tablespoons cake flour, 3/4 teaspoon baking powder, ½ teaspoon baking soda and ½ teaspoon salt in work bowl of food processor and process 5 seconds to blend and set aside.
Place 2 ounces unsweetened chocolate and ¼ cup sugar into the work bowl and process for 1 minute or until chocolate is finely minced.
With the machine running, pour ⅓ cup boiling water through the feed tube and process until the chocolate is melted.
Add 2 large eggs and 1 tablespoon cocoa powder, unsweetened and process for 1 minute.
Add the remaining sugar (1 cup) and process for one minute, stopping once to scrape down the inside of the bowl.
Add ¾ cup butter, unsalted (room temperature), and process for 1 minute more.
Add ½ cup sour cream and 1 tablespoon dark rum. Process for 5 seconds.
Pour the dry ingredients over the chocolate mixture already in the bowl. Combine by turning the machine on and off 3 or 4 times until the flour disappears. Do not over-process.
Pour the batter into prepared pan and spread evenly with a spatula. Bake for 15 minutes at 300˚F minutes, then turn the temperature back to 275˚F and continue cooking for 45 minutes or until a toothpick inserted into the middle of the cake comes out clean. Allow the cake to cool completely before removing from the pan.
Glaze
Put 3 ounces sweet chocolate and ¼ cup powdered sugar in the work bowl and pulse the food processor on and off 3 times to chop the chocolate, then process continuously until it is finely minced.
Boil 2 tablespoons unsalted butter and 2 tablespoons water together. (I do this in the microwave.) With the machine running, pour the hot water and butter through the feed tube and process until the chocolate is melted. Add 2 teaspoons dark rum or vanilla extract and process for 5 more seconds.
Refrigerate the glaze until it begins to thicken; then spread over the surface and sides of the cake.
Notes
To Decorate: Melt 1-ounce white chocolate or white candy coating in a small, zippered, plastic bag. Heat in the microwave until soft and squishy. Ensure the top is sealed.Make a tiny snip across one bottom corner of the bag. Gently squeeze the bag to draw parallel strips across the cake. Use a thin table knife to draw lines perpendicular and parallel across the white lines to make a chevron design. (See the video--this is hard to describe.)Note: I've included many hints in the post above the recipe to ensure your success. If you have questions about an ingredient, please first check the ingredient list in the post.