Line an 8x10-inch tray or tin with baking paper and preheat your oven to 180°C/350°F (or 160℃/320℉ for fan ovens).
Lay the crackers in singular rows until you've covered the base of the tin.
Put the butter and brown sugar into a saucepan. Place over medium heat and stir gently until the butter has melted.
Allow the mixture to start bubbling and from this point, boil it for a further 3-4 minutes, stirring occasionally until it turns slightly darker and thicker (see notes).
Pour the mixture over the crackers in the tin and bake for 5 minutes. In the meantime, melt the milk chocolate.
Remove the tin from the oven (the toffee should be bubbling) and let it sit for exactly 1 minute. Then drizzle the melted chocolate on top and carefully spread it out evenly.
Top with Christmas sprinkles and mini M&Ms.
Allow the crack to cool to room temperature then pop it in the fridge for 2 hours until the chocolate has set fully. Then break it into pieces and enjoy!