In a large bowl, using an electric/stand mixer (or hand whisk, just double/triple the times) combine the butter and both sugars on medium speed for several minutes, until fully combined. Add the egg and vanilla and mix again!
Add the flour and baking powder, plus a pinch of salt, and fold in using a rubber spatula or wooden spoon, until just combined. The dough should feel a little stiff. Add the chocolate and stir together.
Line a baking tray, and scoop the cookie dough into tbsp size balls, roll a little to ensure it’s a ball and sit on the baking tray. We’ll be baking them separately in batches, so don’t worry how spaced apart they are for now! Once all on the tray, refrigerate for at least 30 minutes.
Preheat oven to 350℉/170℃. Line another large baking tray and take 3-4 of the cookie dough balls, place on the tray (spaced at least 2 inches apart).
Bake for 10-12 minutes, until golden at the edges. Repeat for all cookies and leave to cool on a wire rack!