Mix the wet ingredients

• In a large mixing bowl, mix the melted butter, sugar and peanut butter with an electric mixer until smooth and completely incorporated.
• Add the eggs and vanilla extract one at a time, mixing well between each addition.

Combine with the dry ingredients

• In a separate bowl, whisk together the flour, cornstarch, baking soda and baking powder.
• Add the dry ingredients to the wet ingredients and mix just until no dry streaks remain; the dough should be thick.

Chill the dough

• Cover the dough and refrigerate until firm.

Preheat the oven

• Preheat the oven to 165 °C and line two baking sheets with parchment paper.

Shape the cookies

• Scoop the dough out in teaspoon-sized portions and roll each portion into a ball.
• Place two balls next to each other, touching, on the baking sheet, spacing the pairs about 4 cm apart.
• Press down with the tines of a fork widthwise then lengthwise to create a criss cross pattern and flatten the dough.

Bake the cookies

• Bake until the bottoms are deep golden brown.
• Let cool on the sheet for a few minutes, then transfer to a cooling rack to cool completely before filling.

Make the filling

• Mix the peanut butter, powdered sugar, milk and vanilla in a bowl until smooth and completely incorporated.
• If the filling is too thick to pipe, add an additional tablespoon of milk.
• Transfer the filling to a piping bag.

Assemble the sandwich cookies

• Flip a cookie so the flat bottom faces up and pipe a small dollop on each side of the cookie.
• Place another cookie on top to create a sandwich and repeat with the remaining cookies.