Step 1

Place the butter in a medium saucepan, and place over medium heat. Cook until the butter has melted, and then continue to cook, swirling the pan often, until the butter foams and turns golden brown and nutty - this should take 3-4 minutes.

Step 2

Measure out 95g of the brown butter into a medium or large mixing bowl, and set aside for 15-20 minutes to cool so that it doesn't scramble your eggs.

Step 3

When the butter is cool, add the brown sugar and white sugar, and whisk briefly to incorporate.

Step 4

Add the egg and whisk well for 1-2 minutes, or mix using an electric mixer, until the mixture has lightened in colour and has thickened.

Step 5

Add the vanilla and mix well.

Step 6

Add the flour, salt, espresso powder if using, baking powder, and baking soda. Mix to combine with a rubber spatula until there are some flour streaks remaining - this is to ensure you do not over work the dough.

Step 7

Add the chopped chocolate and mix with a spatula to incorporate and finish incorporating the flour.

Step 8

Line a quarter sheet pan or other container with parchment paper. Using a 2 Tbsp cookie scoop, scoop balls of dough (approx 55g) , arranging them on the parchment (they can be touching). Cover with plastic wrap or a lid and chill the dough for at least 30 minutes. See "how to freeze cookie dough" in post for tips on freezing

Step 9

10 minutes into the chilling period, preheat the oven to 350°f / 180°c.  Line two sheet pans with parchment paper.

Step 10

Arrange 6 cookie dough balls onto the pan. If desired, flatten each ball of dough, press more chocolate on the top, and roll into a ball, then space evenly on the pan.

Step 11

Bake the cookies for 11-13 minutes, or until set around the edges and golden brown. Bake for closer to 11 minutes for still gooey inside, or 12-13 for more set cookies.

Step 12

Remove from the oven, and if desired, use a cookie cutter slightly bigger than the cookies to 'scoot' them into a round shape.

Step 13

Sprinkle with flaky sea salt if using. Leave to cool on the pan for 10-15 minutes then transfer to a cooling rack.

Step 14

Repeat the baking process with the remaining 6 balls of dough.

Step 15

Store leftovers in an airtight container at room temperature.