Preheat the oven to 180°C/350°F/Gas Mark 4. Line 2 baking sheets with parchment paper.
In a large bowl, cream together the butter, peanut butter and sugar, using an electric hand whisk/standalone mixer, for approximately 3 minutes until pale and fluffy.
Add the vanilla extract and whisk for a few seconds until combined.
Sieve in the flour and cornflour, and mix until it resembles clumps of damp sand. Add the chocolate pieces/chips and fold through.
Tip the dough out onto a lightly floured work surface and pat it together with your hands. Don’t worry if it feels quite crumbly, just keep binding it together until it holds its shape. Roll the dough into a log shape, about 6” long and cut it into three pieces. Cut each piece in half, and repeat once more until you have 12 pieces of dough.
Roll each piece of dough between your hands to form a ball; again don’t worry if it feels crumbly just pat it together, and place at least 1” apart on the baking sheets. Flatten them down a little using a fork or your hand.
Bake in the preheated oven for 12 minutes or until golden brown. The cookies should still feel slightly squishy to touch; they will firm up as they cool. Remove from the oven and allow them to cool on the baking sheets for 5 minutes before transferring to wire racks to cool completely.
Sandwich a scoop of HB Hazelbrook Farm Banana ice cream between two cookies to serve.