Melt the unsalted butter in a saucepan over medium-low heat for about 5 minutes, stirring constantly until it becomes foamy and takes on a nutty aroma. Remove from heat and let it cool until solid.

Preheat your oven to 350°F (175°C). In a mixing bowl, sift together the all-purpose flour, baking powder, kosher salt, and baking soda.

Once the cooled butter is solid, beat it with dark brown sugar and granulated sugar using an electric mixer on medium speed until light and fluffy. Add the large egg and vanilla extract, blending briefly.

Gradually incorporate the sifted dry ingredients into the wet mixture at low speed, mixing just until combined. Gently fold in the chopped white chocolate, pistachios, and dark chocolate.

Scoop out portions of the cookie dough and roll them into balls. Place each ball in a mini muffin tray, filling about three-quarters full.

Bake the cookie cups for 12-14 minutes or until the edges are set and the centers appear slightly soft. Use a teaspoon to create a well in the center of each cup while they are still warm.

Allow the cookie cups to cool in the tray for about 15 minutes, then run a knife around the edges of each one to loosen them.

Melt the remaining dark chocolate and drizzle it over the cooled cookie cups. Fill each cup generously with pistachio cream.

Refrigerate the completed cups for about 5 minutes to allow the chocolate to set. Store in an airtight container.