Preheat your oven to 350°F (175°C).
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt.
In a large mixing bowl, combine the melted and cooled butter with the brown sugar and white sugar. Beat until well combined and smooth.
Add the egg, egg yolk, and vanilla extract to the butter-sugar mixture. Mix until fully incorporated.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Gently fold in the Sixlets using a spatula.
Using a cookie scoop or tablespoon, drop generous portions of dough onto a lined baking sheet, spacing them about 2 inches apart.
Bake the cookies for 12-15 minutes or until the edges are lightly golden.
Let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack.
