Preheat your oven to 350°F (175°C).
In a large mixing bowl, cream together the softened unsalted butter and light brown sugar until light and fluffy (about 2-3 minutes).
Add the large egg, egg yolk, molasses (if using), and vanilla extract. Mix until fully incorporated.
In another bowl, whisk together the flour, cornstarch, salt, baking soda, and baking powder.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.
Fold in the Demerara sugar and optional English toffee baking bits.
Using a cookie scoop, drop rounded balls of dough onto prepared baking sheets, spacing them about 2 inches apart.
Bake for 10-12 minutes, or until the edges are lightly golden.
Let cool on the baking sheet for 5 minutes before transferring to a cooling rack.
