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Make the crust and streusel: Combine the flour, ground almonds, sugar, and salt. Add the cold cubed butter and rub it into the dry ingredients with your fingertips until crumbly. Add the egg and almond extract, if using, and quickly work everything together into a crumbly shortcrust dough.
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Grease a 10-inch (26 cm) springform pan and line the bottom with parchment paper. Set aside about one-third of the dough for the streusel.
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Press the remaining two-thirds of the dough evenly over the bottom of the pan. There is no need to roll it out. Take small portions of dough, roll them into ropes between your palms, and press them against the inside of the pan to create a rim about 1 1/4 inches (3 cm) high.
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Cover and refrigerate the remaining streusel dough while you prepare the filling.
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Make the poppy seed filling: Add the cold milk, cornstarch, sugar, ground poppy seeds, and salt to a saucepan. Whisk thoroughly while everything is still cold so there are no cornstarch lumps.
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Place over medium heat and bring to a boil, stirring constantly. Continue cooking until the mixture has thickened considerably. Remove from the heat.
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Stir in the orange zest, vanilla, and rum or Amaretto. Let the filling cool for about 10 minutes.
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Meanwhile, drain the mandarin oranges very well. Gently toss them with the lemon juice.
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Pour the poppy seed filling into the prepared crust and smooth the top. Arrange the mandarins evenly over the filling, pressing them very lightly into the surface.
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Crumble the reserved almond streusel dough evenly over the top.
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Bake in a preheated oven at 320°F (160°C), conventional/no fan, for about 60 minutes. Baking at the lower temperature allows the thick poppy seed filling to set gently while the crust and streusel turn golden without becoming too dark.