Preheat oven to 350°F and line a cupcake pan with liners.
In a large bowl, combine cake mix, pudding mix, milk, oil, melted butter, eggs, sour cream, pistachio extract, and vanilla extract. Mix until fully combined, then beat on medium speed for about 1 minute until smooth and slightly fluffy.
Fill liners about ⅔ full. Bake for 16–20 minutes, until a toothpick comes out with a few soft crumbs. Let cool completely before frosting.
For the frosting, beat butter until smooth and creamy. Gradually add powdered sugar and mix until combined. Add 4 tablespoons of heavy cream and the vanilla bean paste, then beat until light and fluffy. Add more cream as needed to reach a soft, pipeable consistency. Mix in green food coloring until you reach your desired shade.
Pipe or spread frosting onto cooled cupcakes. Finish with white candy pearls and top each cupcake with a sparkly straw for that milkshake look.