Whisk the flour, brown sugar, speculaas spices, baking powder, and a pinch of salt together in a large mixing bowl.

Add the cubed cold butter to the flour mixture and rub it in with your fingertips or a pastry blender until the mixture resembles fine breadcrumbs.

Pour in the milk and knead quickly until a smooth dough forms, then wrap it in plastic wrap and chill in the refrigerator for at least 30 minutes.

While the dough chills, prepare the filling by mixing the almond paste with half of the beaten egg and the lemon zest in a medium bowl until smooth and spreadable.

Preheat your oven to 350°F (175°C) and grease an 8-inch square baking pan, lining the bottom with parchment paper.

Divide the chilled dough into two equal portions, rolling the first half out on a lightly floured surface to fit the bottom of your baking pan.

Place the rolled dough into the bottom of the pan, then evenly spread the almond paste mixture over the top, leaving a small border around the edges.

Roll out the second half of the dough and place it carefully over the almond filling, pressing the edges gently to seal the two layers.

Brush the top layer of dough with the remaining beaten egg, arrange the blanched almonds on top in a decorative grid pattern, and brush again with egg wash.

Bake for 35 to 40 minutes until the speculaas is golden-brown and firm to the touch, then let it cool completely in the pan before slicing into squares.