Éclair Cake

Directions

1. Make the pastry cream: Stir together the sugar, yolks, cornstarch, and salt in a heatproof bowl. Combine the milk, cream, and vanilla in a medium saucepan over medium heat and heat until hot. While whisking, slowly stream about half of the hot liquid in the yolk mixture. Pour this tempered yolk mixture into the pot with the rest of the milk mixture (you can pour through a fine-mesh sieve if you're worried about any cooked egg bits). Cook over low to medium-low heat for 4 to 8 minutes, stirring frequently, until bubbling and thickened like pudding. Turn off the heat, then mix in the butter. Strain through a fine-mesh sieve into a heatproof bowl, discarding any clumps caught in the sieve. Cover the pastry cream with plastic wrap to prevent a skin from forming. Refrigerate until cold. (This step can be done up to 2 days in advance.)
2. Make the pâte à choux: In a medium saucepan, combine ½ cup (114 grams) of water, the milk, butter, and salt. Bring to boil over medium heat. Stir in all of the flour with a silicone spatula or wooden spoon. Reduce the heat to low and continue to cook, stirring constantly, until the mixture becomes sticky paste and there’s a visible film of starch on the bottom of the pan, 2 to 3 minutes. Transfer the flour mixture to the bowl of a stand mixer fitted with a paddle attachment. Whisk the eggs in a large liquid measuring cup. With the mixer running on medium speed, add the eggs in slow, steady stream and continue mixing until totally incorporated, about 4 minutes total. Dip the paddle into the batter and lift it up—the batter should form a V shape that eventually breaks away from the batter in the bowl. If the dough is too stiff or pulls away too quickly, add another egg to loosen the consistency.
3. Bake the pâte à choux: Heat the oven to 375°F. Get out an 8-inch square baking dish. Line a sheet pan with parchment paper. Using the baking dish as a guide, trace two squares with pencil on the parchment, then flip the parchment. Pipe or spread the choux evenly inside those two squares (about 320 grams of pâte à choux per square), leaving a ½-inch or so border for the pastry to expand in the oven and fill out the squares. Bake for 30 to 40 minutes, until puffed and golden brown. (Don’t open the oven during the first 25 minutes, otherwise the choux will depuff.) Transfer the baked choux squares to a wire rack to cool completely.
4. Make the ganache: In a medium saucepan over medium-low heat, bring the cream to a simmer. Remove from the heat and add the chocolate and salt. Wait a few minutes, then stir until the chocolate is silky smooth. Let cool until barely warm.
5. Assemble the éclair cake: If needed, use a pair of scissors to trim the border of the choux squares so they can fit inside the baking dish. Place one choux square at the bottom of the baking dish. Stir the pastry cream to smooth out, then evenly spread that on top. Place the second choux square on top. Spread the chocolate ganache on top. Tightly cover the pan and refrigerate for 10 to 24 hours before slicing and serving.