Step 1: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3 minutes using a hand mixer or stand mixer.
Step 2: Beat in the egg and vanilla extract until well combined and smooth.
Step 3: In a separate bowl, whisk together the flour, cornstarch, ground pistachios, and salt.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a soft dough forms. Do not overmix.
Step 5: Wrap the dough in plastic wrap and refrigerate for at least 30 minutes to firm up.
Step 6: Preheat your oven to 325°F (165°C) and line two baking sheets with parchment paper.
Step 7: Scoop 1-tablespoon portions of dough and roll into balls. Place them on the baking sheets about 2 inches apart.
Step 8: Use your thumb to make an indentation in the center of each ball. Bake for 10-12 minutes until the edges are just set but not browned.
Step 9: While still warm, gently re-press the centers if they puffed up. Let the cookies cool completely on a wire rack.
Step 10: Fill each center with a teaspoon of pistachio cream and sprinkle with chopped pistachios before serving.