In a medium bowl, whisk together the melted butter, granulated sugar, and brown sugar until the mixture looks like wet sand.

Crack in the egg and pour in the vanilla extract. Whisk until everything is smooth and glossy, with no streaks of egg left.

Sift in the flour, baking soda, and salt. Gently fold the dry ingredients into the wet mixture with a spatula, stopping when you still see a few streaks of flour.

Add both the dark and milk chocolate chips. Fold them in gently until evenly distributed. Be careful not to overmix!

Using a cookie scoop or spoon, portion the dough into 9–11 pcs. Place them on a plate or tray, cover lightly, and chill in the fridge for at least 1 hour (or overnight for deeper flavor and less spreading).

When ready to bake, preheat your oven to 180°C (350°F). Line a baking tray with parchment paper. Roll the chilled dough into a smooth ball and place them on the tray, spacing them about 2 inches apart to allow room for spreading.

Bake for 9–12 minutes, just until the edges look set and lightly golden, but the centers are still soft and slightly underbaked. This is the secret to keeping your cookies chewy.

Remove the tray from the oven and let the cookies rest for 5–10 minutes. They’ll continue to set as they cool. Then, transfer them to a wire rack to cool completely—or enjoy one warm while the chocolate is still melty.