Make the toffee: combine the butter, sugar, water, vanilla and salt. Place over a medium low heat until all over the sugar is dissolved. Increase the heat to medium and allow the mixture to bubble, swirling the pan occasionally. After about 8 minutes, the mixture will become a deep toffee color and will smell of toffee. Pour it directly onto the baking sheet and spread it as thin as possible. Set aside to firm up.

Brown the butter: Place the butter in a saucepan over medium heat. Allow the butter to melt, then slightly increase the heat and allow the butter to bubble until deeply golden and nutty with brown specks throughout. Transfer the butter to a large bowl and let cool for 10–15 minutes.

Make the cookies: Whisk the brown butter with the dark brown sugar and granulated sugar until glossy. Add the egg, egg yolks, milk, and vanilla and whisk until much lighter and fluffy. Fold in the flour, baking soda, baking powder, and salt until just combined. Fold in the chopped chocolate and toffee pieces, reserving some larger chocolate shards for the tops.

Cover and refrigerate the dough for around 10-15 minutes.

Scoop the cookies onto a parchment lined baking sheet, and place in the freezer overnight.

Preheat the oven to 375°F (190°C) and line baking sheets with parchment paper.

Press extra chocolate pieces onto the outside of the dough balls.

Bake for 12–14 minutes, until the edges are golden and the centers are still soft.

Bang the tray once on the counter immediately after baking, and reshape using a round cookie cutter or small bowl. Sprinkle with flaky sea salt and let cool before serving.