Crisp on the outside with a chewy centre, these moreish cookies have a secret weapon –brown butter. Providing a toasty, nutty depth of flavour, the brown butter tempers the sweetness of the cookies. If you want to amp up the nutty notes, substitute one third of the plain flour for rye flour. If you’re feeling adventurous, switch the salted butter for unsalted and add 1 teaspoon of white miso paste. Miso’s savoury umami notes work wonders with the browned butter and counter the sweetness even further.
Put half the butter in a small pan and melt it over medium heat, swirling often until the butter foams. Continue to cook, swirling and scraping the base of the pan with the heatproof spatula until the butter smells nutty and toasted, and the milk solids have turned a deep golden brown. Pour the browned butter into a large bowl, scraping in all the toasted bits from the bottom of the pan. Set aside to cool for 1–2 minutes, then add the reserved butter cubes, stirring until they melt (this helps to cool down the browned butter).
Add both the sugars to the bowl and beat with an electric hand mixer for about 1 minute, until well combined. Add the egg, egg yolks and vanilla and beat again until the sugar has mostly dissolved – about 2 minutes.
Sift in the flour and bicarbonate of soda, then beat until just combined and almost no pockets of flour remain. Finally, fold through the chocolate, distributing it as evenly as you can through the dough. Set the cookie dough aside to rest for 10 minutes.
Using the ice cream scoop or 2 large spoons, portion the dough into 12 equal-sized balls (of about 60g each). Arrange the dough balls neatly on a lined baking sheet (you need only use one at this stage), then cover and chill for at least 1 hour, or up to 24 hours. (This ‘ageing’ process allows the flour to fully hydrate and hardens the butter in the dough, resulting in a cookie that is well spread and evenly baked.)
Heat the oven to 180°C/160°C fan/350°F/Gas 4. Remove the dough balls from the fridge. You’re going to bake in batches, so arrange 3 or 4 cookies on each of the two remaining lined baking sheets, spacing the balls well apart. Return the remaining balls to the fridge to stay chilled and firm until you’re ready to bake the next batch.
Bake the first batch for 12–14 minutes, until the cookies are well spread and golden around the edges. Remove from the oven and sprinkle with a little flaky salt, if you like, then set the cookies aside to cool on the trays for 10 minutes (they’ll still be very soft at this point, so don’t be tempted to move them too soon). Then, transfer the cookies to a cooling rack to cool completely. Repeat the baking process for a second batch of cookies, until you have baked all 12.