Cut butter into small cubes. Add butter, brown sugar, and granulated sugar to a large bowl or stand mixer with a paddle attachment. Beat until smooth and creamy, like peanut butter. Scrape sides as needed.
Add eggs and vanilla extract. Mix until combined.
Add cornstarch, baking powder, baking soda, and salt. Mix briefly. Add flour and mix just until a dough forms.
Chop Heath bars to the size of chocolate chips and add those along with the chocolate chips to the bowl. Mix until evenly combined.
Heat oven to 350°F. Line a sheet pan with a silicone baking mat.
Scoop dough into 40-gram balls (about 2 packed tablespoons). Place 6 dough balls per tray, spaced at least 2 inches apart.
Freeze shaped dough balls on the tray for 10 minutes.
Bake straight from the freezer for 9 to 12 minutes, until tops no longer look wet. Cookies should be slightly underbaked.
Remove from oven and press extra chocolate chips on top if desired. Let cookies cool on the tray for 5 minutes, then transfer to a cooling rack.
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Notes
Note 1: Use cold butter. Softened or melted butter will change the texture.Storage: Best enjoyed the day they’re baked. Freeze dough balls up to 3 months and bake from frozen, adding 2 to 3 minutes.