Step 1

Melt the butter and dark chocolate in a large heatproof bowl set over a pan of barely simmering water, stirring once or twice, until smooth. Remove from the heat and set aside to cool.

Step 2

Put both sugars, the eggs and vanilla in a mixing bowl and beat with an electric hand mixer for 5 minutes or until creamy, pale and airy (or use a balloon whisk for 5-10 minutes). Add the cooled chocolate/butter mixture and beat for 30 seconds more to combine.

Step 3

Sift the flour, cocoa, bicarb and baking powder into the batter and mix briefly to incorporate. Stir in the chocolate chips, then put the cookie dough in the fridge for 30 minutes to firm up.

Step 4

Heat the oven to 160°C fan/gas Divide the mixture into 16 equal portions, rolling into balls. Push a toffee into the centre of each, then fold the dough around to enclose. Arrange on the trays, at least 5cm apart, flattening each ball slightly. Chill the trays of cookies in the fridge for 10-20 minutes (see Make Ahead).

Step 5

Bake the cookies for 13-15 minutes until just set. Sprinkle with salt flakes, then leave to cool for 15 minutes on the trays. Transfer to a wire rack to cool completely.