Cream the butter and the sugar with an electric mixer until pale and fluffy. Add the egg and mix thoroughly.
Sift together the flour, cocoa, baking powder and bi-carb soda. Gradually add the flour mixture to the creamed mixture to form a dough. Wrap dough in cling film and refrigerate for one hour.
Preheat oven to 170oC. Line three oven trays with baking paper. Lightly flour the work surface and roll out the dough evenly to a thickness of approximately 5mm. Using a 6.5 cm round cookie cutter, cut approximately 40 shapes from the dough, re-rolling the scraps.
Bake in the oven for 10-12 minutes on the middle shelf. Transfer to a wire rack to cool completely.
Meanwhile, remove ice-cream from the freezer and allow to sit at room temperature for 10 mins. Place in a large bowl and mix until soft but not melted. Add softened choc-mint ice-cream to a 22cm x 32cm swiss roll tray and spread to a 2cm thickness. Return to freezer for 2 hours. Repeat with berry ice-cream in a seperate pan.
Remove ice-cream from freezer. Using the same 6.5cm cookie cutter and working quickly, cut out 4 rounds of ice-cream. Return ice-cream tray to freezer. Sandwich between two shortbread biscuits. Continue until all sandwiches are complete. It is a good idea to place finished sandwiches into the freezer to avoid them melting on a warm day.