Preheat oven to 350°F and lightly grease a 9-inch springform pan. Move one of the oven racks to the center of the oven.
In a small saucepan melt chocolate and butter together over medium low heat, stirring until fully combined- then set aside to cool slightly* (1 ¼ cup semisweet chocolate chips, 1 cup butter)
In a different bowl whisk together sugar, heavy cream, vanilla, salt, and eggs (½ cup sugar, ½ cup heavy cream, 1 ½ teaspoon vanilla, pinch of salt, 4 large eggs)
Once the melted chocolate/butter has cooled to being warm, mix into the sugar cream mixture.
Pour the mixture into the prepared spring form pan and bake for 15 minutes at 350°F then lower the oven temperature to 250°F and bake for an additional 30 minutes.
When done baking the truffle cake will be slightly domed and come up to about half the heigh of the springform pan. It will be slightly jiggly. The very center of the cake will not have a dip (that indicated it’s not done baking). Once done baking turn the oven off with the door cracked, leave the cake in the oven allow it to come to room temperature with the oven. This prevents the top from cracking.
Once at room temp the cake will have deflated by half (it’s fine I promise!), refrigerate the cake and make the truffle frosting.