Step 1: Preparing the Dough

1. In a mixing bowl, cream together the butter, granulated sugar, and light brown sugar until light and fluffy.
2. Add the egg yolk and vanilla extract, mixing until well combined.
3. Blend in the pistachio paste until evenly distributed.
4. In a separate bowl, whisk together the flour, baking powder, and salt.
5. Gradually add the dry ingredients to the wet ingredients, stirring until a soft dough forms.
6. Cover and refrigerate the dough for 30 minutes.

Step 2: Baking the Cookies

1. Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. Scoop out portions of dough, rolling them into small balls.
3. Place dough balls on the baking sheet, leaving space between them.
4. Using your thumb or a spoon, press an indentation into the center of each cookie.
5. Bake for 10-12 minutes until edges are lightly golden.
6. Let cookies cool on the baking sheet before transferring to a wire rack.

Step 3: Preparing the Pistachio Cream Filling

1. Melt white chocolate in a microwave-safe bowl, stirring every 30 seconds until smooth.
2. Mix the melted chocolate with pistachio cream and heavy cream, stirring until well combined.
3. Allow the mixture to cool slightly until it thickens to a spreadable consistency.

Step 4: Assembling the Cookies

1. Spoon or pipe a small amount of pistachio cream filling into each cookie indentation.
2. Sprinkle chopped pistachios on top for added texture.
3. Let cookies sit for a few minutes to allow the filling to set.