Beat the butter and both sugars together for about 2 minutes on medium speed.
Add the egg and vanilla extract, mix until well combined.
Add the flour, baking soda and salt. Mix for a few seconds and then add 150 g of ruby chocolate chips, leaving the 30 g for placing on top after.
Scoop 10 balls of cookie dough with an ice cream scooper or make the balls by hand. Place them in the fridge for one hour (or up to 2 days). Check notes for baking right away.
Line a baking tray with parchment paper and preheat the oven to 180°C (350 degrees F).
Place the cookie dough balls on the baking tray at least 10 cm (4 inches) apart as the cookies will spread.
Bake them for 9 minutes, take them out of the oven, place the remaning chocolate on top of the cookies and return to the oven for additional minute or two. Cookies should just slightly start to brown at the edges. They will maybe look not done yet but do not over-bake them!
Take out of the oven and leave to cool.