¾cupunsalted butter((1½ sticks) softened to room temperature)
1cuplight brown sugar(lightly packed)
1large egg yolk
11ouncesbutterscotch chips((1 bag))
½teaspoonvanilla bean paste
Preheat oven to 350 degrees. Line a 9x13 baking pan with parchment paper. (Note 1) Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, add the butter and sugars. Cream together until well blended, about 2 minutes. Add the egg, egg yolk, and vanilla extract. Mix until well incorporated.
Add the flour, cornstarch, baking soda, baking powder, and salt. Mix until just incorporated. Fold in butterscotch morsels. Spread the dough evenly into the prepared pan.
Bake for 21-25 minutes or until a toothpick comes out mostly clean (without wet batter, but may have moist crumbs). Cool completely in the pan.
Remove bars from pan and transfer to a cutting board. Cut into 20-24 squares. Transfer to a cooling rack placed over a sheet pan.
In a bowl, whisk together melted butter, powdered sugar, vanilla bean paste, cardamom, and heavy cream. Mix until the icing is smooth, adding more cream if necessary to reach drizzling consistency. If icing is too thin, add more powdered sugar.
Use a spoon or fork, drizzle the icing over the bars. Let the icing set up before transferring the bars to an airtight container for storing.
Spray the baking pan with cooking spray before lining it with parchment paper. This will prevent the parchment from sliding around while spreading the dough into the pan.
If desired, swap out the butterscotch chips for peanut butter chips, chocolate chips, M&M's, or sprinkles.