Firstly combine the flour, cocoa powder and salt in a bowl, then set aside. In a large bowl beat the butter and sugar together with an electric whisk until light and fluffy. Then add the egg yolk, milk and vanilla and mix until incorporated. Reduce the speed to low, and add the flour mixture until just combined. Wrap the dough in cling film and place into the fridge for an hour to chill until firm.

Preheat the oven to 175C and line two baking trays with baking paper. Whisk the egg white in a small bowl until just frothy and set aside. Finely chop the pecans and place in another small bowl.

Roll the chilled dough into 1-inch balls, dip in the egg whites and roll in the chopped pecans. Evenly place the balls onto the prepared baking trays and using a teaspoon make an indentation in the centre of each ball. I found it helped to dip the spoon into a glass of warm water between each indent.

Bake until just set, about 12-15 minutes and then re-press the existing indentations with the teaspoon. Leave on the baking tray for a few minutes to cool before placing onto baking trays.

For the salted caramel filling, place the toffees and cream into a bowl over a bain-marie and stirring occasionally heat until smooth. This will take about 10 minutes. Carefully fill the cookies with the caramel mixture and sprinkle over sea salt flakes.Then leave to cool completely.