Knead the flour, cornstarch, baking powder, sugar, lemon zest, eggs, salt, and butter into a smooth dough. Refrigerate for 30 minutes.
For the filling, mix sugar, marzipan, and rum into a spreadable mixture using a food processor. If using a hand mixer, it will be more difficult. If the marzipan is hard, heat it in the microwave for 15 seconds to soften. Crumble the sponge fingers and set aside.
For the crumble topping, combine sugar, flour, butter, and cinnamon, and mix until fine crumbs form.
Roll out the dough on a floured work surface into 2 sheets, each approximately 35 cm x 30 cm. Brush each pastry sheet with the marzipan mixture. Spread the raisins, sponge finger crumbs, and hazelnuts over the top. Roll up each sheet from the longer side and refrigerate for 1 hour.
Whisk together the egg yolk and milk. Brush the rolled dough with the egg wash. Sprinkle with the crumble mixture. Cut into wide pieces, yielding about 12 to 16 pieces as desired.
Place the pieces on a baking sheet lined with baking paper. Bake in a preheated oven at 200°C (392°F) for 15 to 20 minutes (use level 3 if baking with gas). Check at 15 minutes to ensure the crumble is not browning too quickly. The pieces should reach a light golden brown color; avoid darker browning as this will dry out the cake.
Remove from the oven, let cool briefly, and dust with powdered sugar.
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