Chocolate Éclair Sponge Cake

Instructions

1. Position a rack in the lower third of the oven and preheat the oven to 350° F. Grease and flour the sides of the pan and line the bottom with parchment paper.
2. Whisk the flour with the baking powder. Sift the mixture three times and return it to the strainer set over a bowl.
3. Whip the eggs, yolks, sugar, and vanilla in the stand mixer at high speed for 2-4 minutes until the batter is light-colored, tripled in volume, and when the whisk is lifted the mixture falls in a thick fluffy rope that dissolves slowly on the surface of the remaining batter foam.
4. While the eggs are beating, heat the milk and butter in a small saucepan until it the butter is melted and the mixture is extremely hot to the touch. (It must be very hot when you add it to the batter.)
5. Remove the bowl from the mixer. With the strainer or sifter, sift one-third of the flour over the eggs. Fold with a large rubber spatula until the flour is almost blended into the batter. Repeat with half of the remaining flour. Repeat with the rest of the flour. Pour all of the hot milk mixture over the batter and fold gently but authoritatively, scraping batter up from the bottom of the bowl and rotating the bowl until the milk and butter are incorporated. Scrape the batter into the pan.
6. Bake 20 to 25 minutes until the cake is a deep golden brown on top and springs back when gently pressed with fingers, or tooth pick comes out clean. Set the pan on a rack to cool.
7. At your convenience, when the cake is warm or completely cool, run a small spatula (I use a small plastic spreader with very thin flexible "blade") around the inside of the pan, pressing against the sides of the pan to avoid tearing the cake. Invert the round the cake and peel off the parchment liner. Turn the cake right side up to finish cooling. The cake should be completely cool before filling, frosting or storing. The cake may be wrapped airtight and stored at room temperature for 2 days, or frozen up to 3 months.
8. To make the vanilla cream: Pour the vanilla into a medium bowl. Set a medium-fine-mesh strainer over the bowl and keep it near the stove for the finished custard.
9. Whisk the sugar and rice flour in a small heavy-bottomed, non-reactive saucepan. Whisk in 4 tablespoons of the milk until blended. Whisk in the egg yolks and the remaining milk. Cook over medium heat, stirring constantly, sweeping the bottom, sides, and corners of the pan to prevent scorching.
10. When the first bubbles appear, set a timer for 5 minutes and continue to cook and stir, adjusting the heat to maintain a gentle simmer.
11. Immediately scrape the custard into the strainer. Stir the custard to pass it through the strainer, but don't press on any bits of cooked egg if any are left behind. Scrape the custard clinging to the underside of the strainer into the bowl as well. Stir to incorporate the vanilla. Let cool for about 30 minutes, then cover with plastic wrap pressed directly against the surface of the custard to prevent a skin from forming. Refrigerate until needed, up to 3 days.
12. For the chocolate glaze: In a medium saucepan, melt the butter. Stir in the sugar and cocoa. Gradually stir in the cream. Heat over medium heat, stirring constantly, until the mixture is smooth and hot but not boiling. Remove from the heat and stir in vanilla and salt.
13. Assemble the cake: Use a serrated knife to split the cake in half horizontally. Set the top layer aside. Spread all of the (chilled!) vanilla cream over the bottom later. Set the top layer on the pastry cream and press it gently to level. Refrigerate the cake for at least a couple of hours and up to one day. If necessary, warm the chocolate glaze in a pan of barely simmering water until it is fluid and pourable but not too thin to coat the cake luxuriously. Set the cake on a baking sheet or directly on a serving platter. Pour the sauce over the cake letting it drip over the sides where it may. Let the cake sit for a few minutes to set the glaze before serving (or refrigerate it serve later).