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Chocolate Éclair Cake

From Sarah: A classic, no-bake recipe, this dessert uses graham crackers to create “cake” layers. I use my pastry cream as a base and add some chocolate to it for a very rich, delicious treat.
From Zoë: I have noted in the Baker's Notes below, the very slight changes I made based on what you will see in the video.
Servings: 24 small squares
Author: Sarah Kieffer

Ingredients

  • 1 14 oz (396g) box graham crackers*

Filling

  • 5 large egg yolks at room temperature
  • 1 1/2 cups (300g) granulated sugar
  • 1/4 cup (25g) Dutch-process cocoa powder
  • 1/2 tsp salt
  • 1/4 cup (28g) cornstarch
  • 1 cup (240g) whole milk
  • 2 1/2 cups (600g) heavy cream
  • 2 Tbsp unsalted butter
  • 2 tsp pure vanilla extract

Glaze

  • 8 oz (226g) semisweet or bittersweet chocolate finely chopped
  • 2/3 cup (160g) heavy cream
  • 2 Tbsp corn syrup

Instructions

Filling

  • In the bowl of a stand mixer fitted with a paddle, beat the egg yolks on low speed. With the mixer running on low speed, slowly add the granulated sugar, followed by the cocoa powder and salt, then increase the speed to medium.
  • Beat the egg-sugar mixture until very thick and pale yellow, about 5 minutes. Scrape down the sides of the bowl and add the cornstarch, then mix on low speed until combined.
  • In a medium heavy-bottom saucepan over medium-low heat, warm the milk and 1 cup (240g) of the heavy cream until just about to summer. Remove the pan from the heat and pour the mixture into a medium liquid measuring cup with a pourable spout.
  • With the mixer running on low speed, very slowly add the hot milk mixture, mixing until completely combined. Transfer the mixture back to the saucepan and cook over medium-low heat, stirring constantly with a wooden spoon, until the pastry cream becomes very thick and begins to boil, 5 to 7 minutes.
  • Switch to a whisk and whisk the mixture until the pastry cream thickens and is glossy and smooth, 3 to 4 minutes. Remove the pan from the heat and strain the pastry cream through a fine-mesh sieve into a medium bowl.
  • Stir in the butter and vanilla. Cover with plastic wrap, making sure the wrap sits directly on top of the cream (this will help keep it from forming a skin). Place in the refrigerator until well chilled, 2 to 3 hours.
  • When the chocolate pastry cream has chilled, in the bowl of a stand mixer fitted with a whisk, beat the remaining 1 1/2 cups (360g) of heavy cream on low speed until small bubbles form, about 30 seconds.
  • Increase the speed to medium and continue beating until the cream is smooth, thick, and nearly doubled in volume, 2 to 3 minutes.
  • Using a spatula, gently fold the whipped cream into the chocolate pastry cream. It will be a little stiff at first, but the cream will incorporate; just keep mixing until it is completely combined.
  • Line the bottom of a 9 by 13 in (23 by 33 cm) baking pan with about a third of the graham crackers in a single layer, breaking up the crackers as needed to fill any spaces.**
  • Top the crackers with half of the chocolate filling. Add another layer of graham crackers over the filling. Cover the top of the filling with the remaining third of the graham crackers. Chill the éclair for at least 6 hours and up to overnight.

Glaze

  • Place the semisweet chocolate in a small heatproof bowl. In a small saucepan over low heat, heat the heavy cream until it is simmering and just about to boil. Pour the cream over the chocolate, cover the bowl with plastic wrap, and let sit for 5 minutes.
  • Remove the plastic wrap, add the corn syrup, and stir the chocolate into the cream until completely smooth.
  • Pour the glaze over the top of the graham cracker layer, right in the center. Using an offset spatula, cover the whole top with the glaze, carefully smoothing it out as you move it to the edges. Place the pan back in the refrigerator and let the éclair chill for at least 2 hours or overnight. When ready to serve, cut into squares. Store the bars in the refrigerator, covered, for up to 2 days.

Notes

Zoë's Baker's Notes:
*I drizzled the graham crackers with bittersweet chocolate. Truth is they were leftover from a S'more party and I used them up in this dessert. You don't need the extra chocolate but if you decide to go for it, I just laid them out on a parchment paper-covered baking sheet, melted 4 ounces / 112g bittersweet chocolate, drizzled the crackers, refrigerated them for about 10 minutes and used them in the recipe. 
**I used a 7x10-inch / 19x25-cm rectangular mold to assemble the "cake" so I ended up with four layers of Graham crackers and three layers of filling. I got 18 pieces instead of 24. Because the cake was smaller I had leftover glaze and used it to pipe decorations on the top.
To get a really clean cut on the bars, use a thin, sharp knife dipped in hot water.