Grease and line a 9inch square cake pan with parchment paper.
Melt the butter, chocolate, and syrup together in a medium bowl over a pan of simmering water. Stirring until smooth and combined. Set aside to cool a little.
Pour a third of the chocolate mixture over the base of the prepared pan. Scatter the plain biscuits, half of the marshmallows and some of the sweets over the base layer of chocolate.
Pour over half the remaining chocolate mixture, then shake the pan a little to allow the chocolate to fill the holes.
Make a second layer with the rest of the biscuits, marshmallow & sweets (reserving a few to scatter over the top), then pour over the remaining chocolate mixture. Shake the pan as before to encourage the chocolate into any holes.
Scatter with the reserved ingredients, then finally scatter over the rainbow sprinkles.
Cool in the fridge for at least 4 hours before cutting into squares.