Preheat oven to 170°C and line 3 large trays with baking paper.Place butter and sugars in a bowl and whisk until smooth. Whisk in egg and vanilla until glossy.
In a separate bowl, whisk flour, bicarb, baking powder, espresso and a pinch of salt flakes together.
Roughly chop chocolate into approximately 1cm pieces.
Add dry ingredients to wet ingredients and mix using a wooden spoon until just combined (there will still be some floury pockets). Fold in the chcolate until evenly incorporated throughout the dough.
Working with 2 tbs of dough at a time, roll into a balls and place on prepared trays. Flatten slightly, leaving 3cm between each cookie, then sprinkle with a pinch of salt flakes.
Bake cookies, 1 tray at a time, for 9 minutes. Open the oven door and carefully tap the tray against the oven rack to release air from the cookies (the cookies will deflate slightly). Bake for a further 4 minutes, then tap again and remove from the oven. The cookies should be golden brown with a crisp edge and the middle just set. Cool for 5 minutes on trays before transferring to a wire rack to cool completely. Sprinkle with salt flakes before serving.
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