Preheat oven to 350℉ and line a cookie sheet with parchment paper.
In a stand mixer or large bowl, use beater to cream butter and sugars together until smooth and creamy.
Add egg and vanilla, beating on low until combined.
Add flour, cornstarch, baking powder and baking soda to mixture and stir until just combined.
Fold in pretzels and Milk Duds.
Scoop 12 evenly sized dough balls onto cookie sheet, spacing 2 -3 inches apart.
Bake in preheated oven for 12-15 minutes or until cookies are browning on the edges and tops but centers are still soft and slightly wet.
Cool on pan for 2 minutes before transferring to a cooling rack to cool completely.
Store cooled cookies in an air-tight container for up to 3 days.