Preheat oven to 180c.
Line 2 baking sheets with baking/parchment paper and set aside.
In a large bowl, beat the butter and sugars together with an electric mixer until light and fluffy.
170 g salted butter, 100 g granulated (white) sugar, 100 g dark brown sugar
Add in egg and vanilla then mix briefly to incorporate.
1 egg, 3 teaspoon vanilla extract
Add dry ingredients and mix by hand (I use a silicone or rubber spatula) until just incorporated.
270 g plain flour (all-purpose flour), 1 tablespoon corn flour, ¾ teaspoon bicarbonate of soda (baking soda/bicarb soda), ½ teaspoon cream of tartar, ¼ teaspoon fine cooking salt
To form the cookies, roll ¼ cup of dough into a ball, then pinch the cookie dough ball roughly in half.
Place one-half of the ball onto the prepared baking sheet, shape the second half into a rough ball and place it on top, pressing down lightly to stabilise it. Repeat with the remaining dough, leaving room for the dough to spread as it bakes.
Bake for 8-10 minutes, or until the cookies are lightly browned at the edges but the centre is still underbaked (you may need to rotate the baking sheets during cooking depending on your oven).
When the cookies come out of the oven, place a large drinking glass (see my notes for alternatives) upside over a cookie and move it in a circle to swirl the cookie around inside the glass and push in the edges.
Allow the cookies to cool on the baking sheet for 5-10 minutes, before carefully transferring them to a wire rack to cool completely.
Sprinkle lightly with flaky sea salt (if using).
sprinkle of flaky sea salt