Cream the Fats and Sugar: In a large mixing bowl, combine the softened butter, powdered sugar, and creamy peanut butter. Use a whisk or an electric mixer on medium speed to beat the mixture until it is smooth, creamy, and pale in color.
Add the Yolk: Add the egg yolk to the creamed butter mixture. Beat gently just until the yolk is fully incorporated. Do not overmix.
Incorporate the Dry Ingredients: Add the all-purpose flour, cornstarch, and the finely ground roasted peanuts into the bowl. Switch to a spatula or use your hands to gently fold and knead the mixture until it forms a cohesive, smooth dough. The dough should feel soft but not sticky.
Roll and Cut: Preheat your oven to 170°C (340°F). Lightly grease your baking sheet with margarine or line it with parchment paper. Place the dough on a flat, clean surface. Using a rolling pin, roll the dough out to an even thickness of about 0.5 cm (1/4 inch). Use cookie cutters to stamp out your desired shapes.
Glaze and Bake: Carefully transfer the cut cookies to the prepared baking sheet, leaving a small gap between each. Using a pastry brush, lightly brush the top of each cookie with melted margarine to create a subtle glaze. Place the baking sheet in the preheated oven.
Bake: Bake at 170°C (340°F) for 20 to 25 minutes, or until the cookies are baked through and the edges turn a faint golden brown.
Cool and Store: Remove from the oven. The cookies will be extremely fragile while hot. Allow them to rest on the baking sheet for at least 10 minutes before transferring them to a wire rack to cool completely. Once completely cooled, store in an airtight container.
