Line a 20cm/8 inch square tin with baking paper and set aside.
Stir in the honey, peanut butter, grapeseed oil and vanilla extract in a saucepan (on low-medium heat) until completely mixed and syrupy. Remove from heat.
Add the cacao powder and milk, making sure that everything is well incorporated.
Place the rice bubbles in a separate bowl. Carefully pour the mixture over the rice bubbles, taking care to coat all the rice bubbles with the peanut butter mixture.
Transfer the rice bubble mixture into the lined tin and ensure that it is spread evenly. Place in refrigerator to set (probably an hour to be safe). Once the peanut butter rice bubble slice has been set, remove from tin and cut up into squares before storing in an air-tight container (it keeps for a few days).