In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy. Beat in the egg and vanilla extract. In a separate bowl, whisk together the flour, baking soda, salt, pistachio pudding mix, and chopped pistachios. Gradually add the dry ingredients to the wet ingredients and mix until a soft dough forms. Stir in white chocolate chips if using. Scoop about 1 tablespoon of dough into each mini muffin cup and gently press down in the center to form a shallow cup. Bake for 10–12 minutes, or until the edges are lightly golden. While still warm, use the back of a spoon to gently re-press the centers if needed. Let cool completely in the pan before removing.