In a large mixing bowl, beat the butter, margarine, palm sugar, powdered sugar, speculaas powder, and salt until well combined. Add the egg yolks and continue mixing until smooth and creamy.
Sift the cake flour, cornstarch, milk powder, and cocoa powder into the butter mixture. Add the chopped canari nuts. Mix until a dough forms and all the ingredients are evenly incorporated. The dough is ready when it holds together easily and can be shaped by hand.
Place the dough between two sheets of plastic wrap and roll it out to about ⅛ inch (2–3 mm) thick. Cut the dough with your favorite cookie cutters and arrange the cookies on a lined baking tray. Decorate each cookie with sliced nuts.
Bake in a preheated oven at 284°F (140°C) for 35 minutes, or until crisp. Transfer the cookies to a cooling rack and let them cool completely before serving. Enjoy!
Notes
Chop the nuts finely, but not too fine - For the best texture, chop the canari nuts finely while leaving some small pieces intact. This gives the cookies a pleasant crunch and allows you to taste the nuts in every bite.
Don't overmix the dough - Mix the dough just until everything comes together. Overmixing can make the cookies tougher and less crisp after baking.
Roll the dough evenly - Try to keep the dough at an even thickness throughout. This helps the cookies bake uniformly so you won't end up with some cookies overbaked while others remain soft.
Chill the dough if it becomes too soft - If the dough becomes sticky or difficult to handle, refrigerate it for 15–30 minutes before rolling. Chilled dough is easier to roll out and cut into neat shapes.
Let the cookies cool completely - The cookies will continue to crisp up as they cool. Allow them to cool completely on a wire rack before storing them in an airtight container.