Preparation Steps

• Make the pastry cream base: whisk the granulated sugar and egg yolks in a medium bowl until the mixture is pale and slightly thickened. Stir in the cornstarch and flour until smooth.
• Warm the milk in a small saucepan over medium heat until it just begins to steam and tiny bubbles form at the edge (do not boil). Remove from heat.
• Temper the egg mixture: slowly pour about one-third of the hot milk into the yolks while whisking constantly, then pour the tempered yolk mix back into the saucepan with the remaining milk. Return the pan to medium-low heat and cook, whisking continuously, until the mixture thickens and holds a ribbon when the whisk is lifted, about 2–4 minutes.
• Off the heat, whisk in the butter, vanilla, and lemon zest until completely incorporated. Press plastic wrap directly onto the surface to prevent a skin from forming and chill until cool, at least 30 minutes.
• When the pastry cream is cold, loosen it with a few quick whisks to make it smooth. In a separate bowl, whip 1/2 cup very cold heavy cream to soft peaks, then fold it gently into the pastry cream in three additions using a rubber spatula to keep the texture light.
• For the mascarpone chantilly: in a chilled bowl, combine the mascarpone, 1 cup very cold heavy cream, powdered sugar, and vanilla. Whip on medium speed until the mixture reaches a fluffy, pipeable consistency—stop as soon as it moves past soft peaks (about 2–3 minutes) to avoid overbeating.
• Cut the angel food cake into roughly 1-inch cubes and set up a 9x13-inch (23x33 cm) baking dish or sheet pan lined with parchment for easy removal.
• Assemble the cake: spread half of the cake cubes in an even layer across the bottom of the pan. Spoon about two-thirds of the blueberry filling over the cake, leaving roughly 2 tablespoons aside for the top.
• Add the remaining cake cubes in a second layer and press lightly to even them. Smooth the cooled vanilla pastry cream over the cake layer in an even sheet using an offset spatula.
• Spread the mascarpone chantilly over the pastry cream. Dollop the reserved blueberry filling on top and use a knife or small spatula to swirl it gently into the cream for marbled ribbons.
• Cover and refrigerate for at least 4 hours to firm up (overnight is best for clean slices). Just before serving, sprinkle the toasted almond slivers over the top for crunch.
• To serve, cut the set cake into 12 squares and keep refrigerated. Leftovers keep well in an airtight container for up to 2 days.