Put the chocolate in a heatproof bowl. Heat the cream over medium heat until it forms little bubbles around the edge of the pan. Pour the cream over the chocolate and let it sit for 5 minutes, then add the butter and sugar and stir until completely smooth. Set aside to cool.
Make the cake
Preheat oven to 350 (175 C) degrees with rack placed in the center.
Oil a 9-inch savarin mold or plain tube pan with oil or cooking spray.
Sift the flour, cornstarch, salt and cocoa powder through a fine strainer set over a mixing bowl.
Whip the egg whites in a large bowl (by hand or with an electric mixer with the whip attachment) until they form soft peaks.
In another mixing bowl, whisk the egg yolks, sugar, yogurt and vanilla until smooth and airy. Slowly add the oil while mixing to incorporate. Add the flour mixture to the egg mixture and combine with a wooden spoon or spatula until the flour is no longer visible. Add the egg whites and fold gently until incorporated.
Pour the batter into the pan (it will almost reach the top) and place in the oven. Bake 30-35 minutes, until the edges of the cake begin to pull away from the sides and a toothpick inserted in the middle comes out clean. The cake will crack along the middle — that's what you want.
Cool in the pan 10 minutes, then gently invert onto a cooling rack. Cool completely.
Put the cake cracked side up on a large plate or serving platter. Pour the glaze over the cake. Dump the berries in the middle of the cake, then slice and serve.
Notes
This cake makes small, snack-size slices and disappears pretty quickly! It will keep well at room temperature for a day or two (without the berries).
The shallow tube pan to use for baking a ring cake is called a savarin pan. It's inexpensive and easy to find online, but it may not be a tool you have in your kitchen. Bake the cake in a plain tube cake pan or a fluted tube pan, and be sure to oil it thoroughly.
The cake can be mixed by entirely by hand, but I find it easiest to use an electric mixer to whip the egg whites.
Let the cake cool before adding the glaze — it will set faster.
You can substitute sour cream or buttermilk for the yogurt.
This recipe was inspired by Nigella's recipe for "Yogurt Carton Cake" in Nigellissima.