Line an 8-inch square tin with baking paper, greasing with butter to prevent sticking.

In a saucepan, melt butter, golden syrup, dark chocolate, and milk chocolate over low heat, stirring until smooth (5–7 minutes).

Let chocolate mixture cool for 10 minutes.

In a mixing bowl, combine crushed biscuits, mini marshmallows, and Maltesers.

Pour two-thirds of the cooled chocolate mixture over the fillings and stir gently.

Transfer the mixture to the prepared tin and compact it, filling gaps with remaining chocolate mixture.

Scatter extra Maltesers on top and press some into the mixture.

Chill in the refrigerator for at least 3-4 hours or overnight.

Once set, remove from the tin using the overhang and slice into bars.

Store leftovers in an airtight container at room temperature for up to two weeks.