Preheat your oven to 350°F (175°C).
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until smooth.
Beat in the egg and egg yolk until well combined.
In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Stir in the chocolate chips or filling.
Drop spoonfuls of dough onto a baking sheet lined with parchment paper, leaving space between each cookie.
Bake for 10-12 minutes, or until the edges are lightly golden.
Let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely. Enjoy your chocolate-filled cookies!