Preheat the oven to 350ºF. Line an 8-inch square pan with parchment paper and spray with nonstick cooking spray. Make sure to leave extra overhang of the parchment paper for easy removal.

In a stand mixer fitted with the paddle attachment, cream the butter, brown sugar, and salt together until creamy, about 3 minutes. Add the flour and cornstarch and mix on low speed until a crumbly mixture forms.

Press the dough evenly into the prepared pan. Pierce crust with a fork in about a dozen places. Bake for 13-15 minutes, or until edges are lightly browned and center is set. Cool completely.

In a stand mixer fitted with the paddle attachment, beat the peanut butter, butter, vanilla, and salt together until smooth and creamy, about 2 minutes. Add the sugar and continue beating until combined. Spread filling over shortbread using a spatula, knife, or offset knife. Chill in the refrigerator for at least 4 hours.

Place the chocolate and coconut oil in a microwave-safe bowl and heat in the microwave until smooth and melted. Line a baking sheet with wax paper.

Remove the now-chilled bars from the pan by sliding a knife along the edges and using the parchment sides to lift the bars out of the pan and onto a cutting board. Using a sharp knife, cut the bars into 16 squares.

Using two forks, dip the bars into the melted chocolate until completely coated, then gently tap off the excess chocolate. Place onto the wax paper-lined baking sheet. Repeat with the remaining bars.

Refrigerate bars until chocolate is set, about 15 minutes.