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½ cup of the hot mixture in a slow stream into the egg yolk mixture while whisking quickly to temper the egg yolks. Pour the egg yolk mixture slowly back into the saucepan while stirring constantly. Return the pan to medium-high heat, stirring constantly, until the mixture thickens and begins to boil. (Once the mixture begins to thicken, it must be handled gently so the cornstarch won't lose its thickening power.)
Remove the pan from the heat and add the vanilla. Add butter if you did not use heavy cream. Stir gently until the butter is melted and combined. Strain the custard through a sieve into a bowl to ensure a smooth custard. (Use a rubber spatula to push the custard through the sieve.) Cover with plastic wrap and refrigerate until cold.
To assemble the cake, place one cake layer top-side down on a cardboard round. Spread the custard over the cake until within ½ inch of the border. Top with the other cake layer, top-side up. Dust the top of the cake generously with the confectioners' sugar. Keep refrigerated, but bring to room temperature before serving. Serve with whipped cream and have it with tea.
    Serves 8
    Being that Sirius was sending letters to Harry via tropical birds, it follows that the cake he sent him would contain tropical flavors, like citrus and mango.
Citrus Sandwich Cake with Mango Filling (from Sirius)
    Citrus Cake
    2 cups cake flour
    1 cup granulated sugar, divided
    1 teaspoon baking powder
    ¼ teaspoon salt
    1½ sticks (12 tablespoons) butter, at room temperature
    ½ cup whole milk
    3 large egg yolks, at room temperature
    Grated zest and juice of 1 lemon
    Grated zest and juice of 1 orange
    1 teaspoon pure vanilla extract
    3 large egg whites, at room temperature
    Mango Filling
    3 ripe mangoes, peeled and cut into chunks
    ½ cup granulated sugar
    ½ cup water Pinch salt
    2 tablespoons cornstarch
    3 large egg yolks
    ¼ stick (2 tablespoons) butter
    Juice of 1 lemon
    Whipped Cream Frosting
    2 cups heavy cream
    2/3 cup confectioners' sugar
    2 teaspoons pure vanilla extract
    A few drops yellow food coloring
Preheat the oven to 350°F. Grease and flour two 8-inch round cake pans and line the bottoms with parchment paper.
To make the cake, whisk together the flour, ½ cup of the sugar, baking powder, and salt. Add the butter and beat until the mixture resembles yellow crumbs. Add the milk, yolks, grated zest and juice of lemon and orange, and vanilla and beat until smooth, using the whisk attachment.
In a separate clean bowl with clean whisk, beat the egg whites until soft mounds form. Continue beating while adding the remaining ½ cup sugar gradually. Beat until stiff but still glossy. Whisk ¼ of the beaten egg whites into the batter to lighten it, then fold in the rest of the egg whites using a rubber spatula. Divide the batter evenly between the two pans and bake for 20 to 25 minutes, until the cakes are spotty brown and feel firm when touched lightly in the center. Do not overbake or the cakes will be dry. Cool in the pans, then invert the pans to remove the layers. If desired, the layers can be wrapped in plastic and frozen until needed.
To make the filling, process the mangoes in a food processor or blender until smooth, then push through a sieve with a rubber spatula, pushing down to extract as much juice as possible. Discard the pulp. Transfer the sieved mango to a medium saucepan. Add the sugar, water, salt, and cornstarch and stir to dissolve. Cook over medium heat until warm, stirring constantly. Whisk in the egg yolks and cook, stirring constantly, until thickened and bubbling. Remove from the heat, add the butter and lemon juice, and stir until melted and combined. Transfer the filling to a bowl, cover the surface directly with plastic wrap, and cool to room temperature. Refrigerate until needed, up to 2 days.
Make the frosting just before you're ready to assemble the cake. Whip the frosting ingredients together until stiff peaks form.
To assemble the cake, split the cake layers in half. Place