Pre-heat your oven to 160C Fan/350F/Gas Mark 4, grease a 26cm skillet pan with butter
Using a stand mixer with the paddle attachment, mix the butter and both sugars together until fluffy and paler in colour. This will take about 3 minutes
Add the egg and vanilla extract and mix in
Add the plain flour, baking powder and salt and mix them in, ony mix until just combined, be careful not to over mix
Add the chocolate chips and mix briefly, just to evenly distribute them
Using your hands, press the dough into the skillet pan until level
Bake for 25-30 minutes until lightly golden all over
Cool for 10 minutes (just so you don't burn your mouth!) then serve immediately
Once fully cool, you can cover it or transfer it to an airtight container, and store it at room temperature for 3 days