This cake manages to be both delicate and densely nutty at the same time. Swathed in a fluffy custard frosting, it's unusual and perfect for anyone who loves almond flavor. For tips on how to DIY caster sugar, see this article.
First, make the custard for the frosting: Whisk together the cream and sugar in a heavy-bottomed saucepan and cook over medium-low heat until just barely simmering.
In a large heatproof bowl, whisk together the egg yolks, cornstarch, and vanilla extract.
Slowly pour the cream mixture into the egg yolk mixture, whisking constantly so as to not cook the eggs. Once combined, transfer the custard back into the pot and cook over medium-low heat (whisking constantly) until the mixture thickens and coats the back of a spoon. This will take about 5 minutes, or more depending on the heat level. Once thickened, transfer the custard back to the heatproof bowl, place a piece of plastic wrap over the surface (so a skin doesn't form), and let cool fully in the fridge.
While the custard cools, make the cake. Preheat the oven to 350℉ and grease and flour (or line with parchment) two 8" round cake pans.
In a clean bowl of a stand mixer with the whisk attachment, beat the egg whites until stiff. Scatter half of the sugar over the egg whites and whisk to combine.
Whisk in half of the almond flour (the mixture will deflate slightly at this point; that's fine).
Using a spatula, fold in the remaining sugar, almond flour, almond extract, and all-purpose flour. Divide the batter between the prepared pans and bake for 25 minutes. Remove from the oven and let cool for 10 minutes before turning out onto a wire rack to cool completely before frosting.
Finish the frosting by beating the softened butter until very light and fluffy. Slowly add the cooled custard with a pinch of salt and beat until spreadable. If it seems too soft, chill in the fridge for 20 minutes then try beating it again.
To assemble the cake, spread a layer of frosting over the first cake layer, top with the second cake layer, then frost the tops and sides.