Heat the oven to 180C/160C fan/gas 4 and line two large baking sheets with baking parchment. Beat the butter and sugars together in a large bowl using an electric whisk, or in a stand mixer until just combined. Crack in the egg and beat again. Stir through the vanilla, flour, bicarb, chocolate chunks and 1/4 tsp fine sea salt.

Roll the mixture into 10 even-sized balls between the palms of your hands. Arrange over the prepared baking sheets and bake for 10-12 mins until golden brown at the edges. Leave to cool slightly on the sheets, then transfer to a wire rack and leave to cool completely, or eat warm with ice cream.