Position a rack in the center of the oven and preheat to 350 degrees F. Line a half-sheet pan with a silicone baking mat, or lightly butter the pan and line the bottom with parchment paper.
Sift the flour, sugar, cocoa, baking powder, and salt together. Combine the cream, zest, and vanilla in the bowl of a heavy-duty stand mixer. Attach the bowl to the mixer and fit with the whisk attachment. Whip on medium speed until the cream holds its shape, just before soft peaks begin to form. Gradually beat in the eggs-the mixture will thin somewhat. Change to the paddle attachment. On low speed, add the flour mixture, and beat until smooth, scraping down the sides of the bowl, about 1 minute. Because the cream is chilled, the batter thickens to the consistency of very stiff frosting. Distribute the batter in 5 large dollops in the pan, 1 in each corner and 1 in the center. Spread evenly with an offset metal spatula.
Bake until the cake springs back when pressed gently with your finger in the center, about 15 minutes. Transfer to a wire rack and let cool completely. Run a knife around the inside of the half-sheet pan to loosen the cake. Place a large piece of parchment paper over the cake, then top with a large cutting board. Invert the pan and board together, and remove the pan and mat. Cover the cake with plastic wrap and refrigerate to chill and firm, at least 1 hour and up to 1 day before using.
Use this cake recipe as the base for Sarabeth's Chocolate Truffle Cake.
The cream in this cake makes a very tender crumb that can break when handling. Refrigerate the unmolded sheet cake until the cake is chilled and firm, and the cut rounds will be sturdier and ready for stacking.
Recipe courtesy of Sarabeth Levine, from Sarabeth's Bakery: From My Hands to Yours