Cream together the dairy-free butter with the brown and caster sugars. Stir in the custard powder and milk.
Sift in both flours, baking powder, bicarbonate of soda and salt. Stir to form a soft but thick dough. Stir in the chocolate chips so they are evenly mixed through the dough. Cover the dough or place in a Tupperware box and refrigerate for at least 12 hours.
Preheat the oven to 170 degrees centigrade. Line 2 cookie sheets with parchment.
Roll the dough into 8 large balls or 16 small and place well-spaced onto the cookie sheets. Press the extra chocolate into the top of the balls and sprinkle with a little flaky sea salt
Bake for 14-18 minutes for the large cookies (10-12 minutes for the small) until they are lightly golden around the edges. Cool briefly on the cookie sheets until set and then transfer to wire racks. These keep really well for a few days in an airtight container
Or roll the dough into balls and then freeze: to bake from frozen simply add a minute or two to the baking time
