Beat the butter and sugar together in a bowl using an electric whisk or using a stand mixer, until the mixture is light and fluffy. Beat in the vanilla, egg and milk until just combined. Sift over the cocoa, then stir in the flour, bicarb and 1 tsp sea salt flakes until a dough forms. Fold in the white chocolate chunks, then chill the dough for at least 3 hrs, or up to 24 hrs.

Heat the oven to 180C/160C fan/gas 4 and line two large baking sheets with baking parchment. Divide the dough into 12 portions (weighing about 60g each) and roll each one into a ball. The unbaked dough balls can be frozen for up to three months (see tip, below). Arrange six balls over each baking sheet, leaving space between for spreading. Bake for 10 mins, remove from the oven and sprinkle over more sea salt. To neaten your cookies, you can put a larger round biscuit cutter or glass over the warm cookies on the baking tray, and gently swirl in a circular motion to round out the edge of the cookies.

Bake for 5 mins more for soft cookies, or if you prefer a firmer cookie, add a few extra minutes to the baking time. Leave to cool on the baking sheets for 10 mins, then transfer to a wire rack to cool until just warm, or leave to cool completely. Once completely cool, will keep in an airtight tin for a few days.