Step 1

Preheat the oven to 180°C, gas mark 4 and line a 20x25cm baking tin with baking parchment.

Step 2

Put the butter and both the sugars into a large mixing bowl and beat together with an electric hand mixer until light and fluffy. Add the eggs and vanilla extract and mix together with a spoon until just combined. Fold in the flour, bicarbonate of soda and 1 heaped tsp fine salt, until only a few streaks of flour remain, then fold in 350g of the chocolate chips until evenly distributed.

Step 3

Scoop the cookie dough into your prepared tin and smooth it out to the edges, then scatter the remaining chocolate chips on top.

Step 4

Bake in the oven for 20-25 minutes until the edges are golden and there is still a very slight wobble in the middle. Remove from the oven and while still warm, sprinkle over the sea salt flakes, if using (this is optional but recommended!). Leave to cool completely in the tin, then cut into 16 bars.

Step 5

These are best eaten on the day of baking or the day after, but can be stored in an airtight container at room temperature or in the refrigerator for up to 5 days. Feel free to reheat the bars to make them warm and gooey again.