Melt butter in a microwave-safe bowl, then refrigerate for 3–5 minutes to cool to room temperature. Once cooled, scrape the butter into a large bowl with a spatula.
Add brown sugar to the cooled butter. Whisk together for 1 minute, then add the whole egg, egg yolk, molasses, and vanilla. Whisk briskly until the mixture is very creamy and emulsified.
Add the cornstarch, salt, baking soda, and baking powder directly to the wet ingredients and whisk to combine. Add flour (see note 2). Mix with a wooden spoon or hand mixer until no dry streaks remain. If using toffee bits, gently fold them in. Cover the dough and chill in the fridge for 30 minutes.
Measure dough into 2-tablespoon (40-gram) balls and roll in Demerara or granulated sugar. Place on a plate, cover, and refrigerate for 45–60 minutes.
Preheat oven to 350°F. Space dough balls on a lined baking sheet (6 per sheet), and bake for 9–14 minutes. (I love these best at 10 minutes.) Cool on the pan for 3–5 minutes, then transfer to a cooling rack.
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Notes
Note 1: Molasses deepens the brown sugar flavor, but the cookies are still delicious without it.Note 2: If you have a food scale, use it here! You’ll need 415 grams of flour.Note 3: Alternatively, roll the cookies in turbinado sugar, sparkling sugar, or granulated (plain) sugar.Nutrition Note: Nutritional information does not include the optional toffee chips.Storage: Store baked cookies at room temperature in an airtight container for up to 1 week. To freeze the dough, place unbaked cookie dough balls on a pan and freeze for one hour. Once solid, transfer them to a bag and freeze for up to 3 months. Bake directly from frozen, adding a few extra minutes to the baking time.