Add butter, sugar, and almond paste to a bowl and mix until combined. Add egg, ground cinnamon, ground cardamom, ground clove, ground nutmeg, and salt and mix again. Sift flour into the bowl and stir a last time until a smooth dough forms. Wrap in plastic wrap and let rest in the fridge for approx. 2 hrs.
Preheat oven to 190°C/375°F. Line two baking sheets with parchment paper, dust with some flour, and spread sliced almonds evenly on it.
Roll out dough on a silicone mat or between two sheets of plastic wrap. It should be approx. 0.5-cm/0.2-in. thick. Either using a cookie cutter or a special speculaas rolling pin, cut out cookies and transfer them onto the prepared baking sheets. Bake at 190°C/375°F for approx. 10 min., or until golden brown. Remove from oven and let them cool down completely on a wire rack. Enjoy!