Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper or silicone liners.
Whisk together the flour, baking soda, baking powder, and salt. Set aside.
In a large mixing bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy. (This will take about 2-3 minutes.) Beat in the egg, egg yolk, and vanilla extract until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined or a few small streaks of flour remain. Be careful not to over-mix.
Gently fold in the toffee pieces.
Using 2 tablespoons of dough at a time, roll the dough into balls and place on the prepared baking sheets, spacing them about 3 inches apart. (These cookies will spread quite a bit, so leave plenty of room.) If you like, add more Heath bits to the tops by sprinkling them on top or dipping each cookie dough ball into the toffee pieces.
Bake one pan at a time for 10-12 minutes, or until the edges are golden brown and the centers look slightly underbaked. The cookies will continue to set as they cool.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
Notes
Store in an airtight container at room temperature up to 5 days.
For tips, step-by-step photos, and extra details, be sure to read the full post above before starting. It’s packed with helpful info to answer common questions!