In a stand mixer, combine melted butter, granulated sugar, light brown sugar, and vanilla.
Add the egg and peanut butter, mixing until incorporated.
Add the flour, cornstarch, baking soda, baking powder, and salt, mixing until combined.
Scoop 16 even dough balls (2-2.5 tablespoons each) and place them on a parchment-lined baking sheet.
Chill the dough for 30 minutes.
Preheat the oven to 350°F.
Roll the chilled dough in granulated sugar, then flatten each ball with a fork in a criss-cross pattern.
Bake for 9-10 minutes or until the edges are set.
Let cool on the sheet for 5 minutes, then transfer to a rack.
Once cooled, freeze for 30-60 minutes before dipping.
Melt the chocolate wafers and dip each cookie, placing it on a parchment-lined sheet to set.