Set your oven to 175–180 °C (350–355 °F) and line a small baking tray with parchment so the cookies don’t stick.
In a small mixing bowl, beat the softened butter with the brown sugar and granulated sugar until the mixture looks creamy and well combined.
Add the egg yolk and vanilla extract, then mix again until the mixture is smooth and uniform.
In another bowl, whisk together the flour, baking soda, and salt so the leavening is evenly distributed.
Tip the dry ingredients into the wet mixture and gently stir until a soft dough forms and no dry streaks remain.
Fold in the chocolate chips so they are spread throughout the dough.
Scoop the dough into 6–8 portions on the prepared tray, leaving space between each one for spreading.
Bake for about 8–11 minutes, until the edges are lightly golden and the centers still look slightly soft.
Let the cookies cool on the tray for a few minutes, then transfer them to a rack to cool a little more before serving.
Once completely cool, store any leftovers in an airtight container at room temperature and enjoy within 1–2 days.