Line an approximately 28 × 18cm brownie or slice tin with baking paper.
Add the butter, honey, vanilla essence, cocoa powder and mini marshmallows to a heavy-based saucepan.
Heat over medium heat, stirring regularly, until everything has melted and combined.
Once melted, bring the mixture to a gentle bubble and cook for 2–3 minutes, stirring frequently so it doesn't catch on the bottom.
Remove from the heat. Add the rice bubbles and stir until they're evenly coated in the chocolate mixture.
Transfer to the prepared tin and press the mixture down firmly and evenly.
Sprinkle with chocolate chips or festive sprinkles, if using, and gently press them into the top.
Refrigerate for 2–3 hours, or until completely set.
Remove from the tin and cut into approximately 16–20 squares.