I made this cake as part of a brownie/mousse/cake cake (known as Chocolate Overdose). I followed all the directions (below). The cake stuck some to the sides and sunk a tiny bit in the middle. Then when I inverted it, it completely fell apart. Basically broke into 4 pieces. 
What did I do wrong?
Here is the cake recipe:
2 oz. unsweetened chocolate, coarsely chopped
1/8 cup Dutch-processed cocoa
1/4 cup hot water
7/8 cup sugar, divided
7/8 cups unbleached all-purpose flour
3/4 tsp. baking soda
1/2 tsp. salt
1/2 cup buttermilk
1 tsp. vanilla extract
2 large eggs
1 large egg yolk
6 tbsp. unsalted butter, soft
To make the cake, line the bottom of a 9-inch round cake pan with a round of parchment paper. Grease and flour the sides of the pan. Combine the chocolate, cocoa and hot water in a medium heatproof bowl; set the bowl over a saucepan containing 1-inch of simmering water and stir with a rubber spatula until chocolate is melted, about 2 minutes. (Note: at this point, my mixture was pretty thick and I thought I had ruined it, but mixing in the sugar was like magic and it made it smooth and liquidy again.) Add 1/4 cup of sugar to the chocolate mixture and stir until thick and glossy, 1 to 2 minutes. Remove bowl from heat and set aside to cool.
Whisk flour, baking soda and salt in a medium bowl; set aside. Combine buttermilk and vanilla in a small bowl.
In the bowl of a stand mixer fitted with the whisk attachment, whisk the eggs and yolks on medium-low speed until combined, about 10 seconds. Add remaining sugar, increase speed to high, and whisk until fluffy and lightened in color, 2 to 3 minutes. Replace whisk with paddle attachment.
Add the cooled chocolate mixture to the egg/sugar mixture and mix on medium speed until thoroughly incorporated, 30-45 seconds, pausing to scrape down sides of bowl with rubber spatula as needed. Add softened butter one tablespoon at a time, mixing about 10 seconds after each addition. Add about one third of the flour mixture followed by half of the buttermilk mixture, mixing until incorporated after each addition (about 15 seconds). Repeat using half of remaining flour mixture and all of remaining buttermilk mixture (batter may appear separated). Scrape down the sides of the bowl and add remaining flour mixture; mix at medium-low speed until batter is thoroughly combined, about 15 seconds. Remove bowl from mixer and fold batter once or twice with rubber spatula to incorporate any remaining flour. Pour into prepared pan; smooth batter to edges of pan with an offset spatula.
Bake at 350 for 30 minutes, or until a toothpick inserted in the center of each comes out clean. Transfer the pan Allow the cake to cool in the pan for 15 minutes, then invert onto the wire rack and allow to cool completely.
