Preheat oven to 220℃.
Line 2 baking sheets with parchment paper and set aside.
Whisk together plain flour, baking powder and salt, then set aside.
220 g plain flour (all-purpose flour), ½ teaspoon baking powder, ¼ teaspoon salt
In a large bowl, combine butter and brown sugar. Using an electric mixer, beat together until light and fluffy.
150 g salted butter, 160 g light brown sugar
Add the egg and vanilla and beat until just combined.
1 large egg, 2½ teaspoons vanilla extract
Swap to a silicone spatula (or similar) and stir in chocolate chips, then fold in flour mixture.
150 g chocolate chips
Roll dough into balls (roughly ¼ cup of mixture), place on prepared baking sheet, leaving space for cookies to spread. Bake for 5 minutes (set a timer).
For consistent results, bake one sheet at a time (see my notes).
Once baked, remove from oven and immediately place a large drinking glass (see my notes) upside down over each cookie and move it in a circle to spin the cookie around inside the glass and push in the edges.
Leave the cookies on the baking sheet for 20 - 30 minutes to firm up and set.