Beat the butter and sugars in a stand mixer fitted with the paddle attachment until light and fluffy.
Add egg and vanilla, beating well, then beat in peanut butter and Nutella. Turn off the mixer, then use your hands to incorporate the flour, bicarb, cocoa, chocolate chips and 1/2 tsp salt flakes.
Place a sheet of baking paper on a clean work surface, then place half the dough on the paper and form a rough 6cm-diameter log. Roll the paper tightly to enclose, securing the ends and twisting to tighten. Enclose it in plastic wrap. Repeat with the remaining dough. Freeze for up to 3 months or, if using immediately, chill for 1 hour to firm.
When ready to bake, preheat the oven to 180°C. Grease a baking tray and line it with baking paper.
Slice off as many 1cm-thick biscuits as you need at a time. Spread over the tray, leaving 2cm between biscuits. Bake biscuits straight from the fridge or freezer for 12 minutes or until a biscuit moves on the tray after being pushed gently with your finger. Cool on tray.
If making ice cream sandwiches, place small scoops of ice cream on a chilled baking tray lined with baking paper. With a piece of baking paper on top and working in batches, flatten with your hand to the size of the biscuits. (If necessary, return flattened ice cream to the freezer while preparing the remaining ice cream.)
Sandwich each piece of ice cream with two biscuits and serve immediately or freeze to serve within 4-6 hours.
