Lemon-Vanilla Karpatka with Crunchy Topping

Everybody loves this old-fashioned cake. It starts with two simple choux pastry layers, which puff dramatically, resembling the Polish Karpaty mountains. These are filled with butter pastry cream, which I like to improve with tangy lemon zest and vanilla. But choux pastry gets soggy in time. To keep it crunchy for longer, I cover the cake with a sugar-butter-flour topping; this also makes it look like a bird’s-eye view of the mountain forest. I fell in love with karpatka in childhood