Preheat oven to 350 degrees F. Line baking sheet with parchment paper.

In bowl of stand mixer, beat together butter and sugar until light and fluffy, about 3 minutes.

Add egg yolk, pistachio extract and vanilla extract and blend until combined.

Slowly add flour and cacao powder and blend on low until stiff dough forms. (Note: Add small bit of milk of choice if crumbly.)

Roll dough into silver dollar-sized balls and arrange on prepared baking sheet.

Using thumb or measuring spoon, press balls in center to flatten slightly and form indentation.

Bake for 8-10 minutes, until edges are puffed and slightly darkened.

In microwave-safe bowl, warm white chocolate in 30-second increments until melted.

Stir pistachio butter into white chocolate until smooth.

Fill cookie indents with mixture, sprinkle on pistachios and sea salt and cool cookies on wire rack until centers are set.