Melt chocolate, butter and golden syrup together over low heat.
Put digestive biscuits in a ziptop bag. Bash with a rolling pin until you have a mixture of coarse and fine crumbs.
Remove 1/2 cup of the melted chocolate mixture and set aside. Add biscuit crumbs and marshmallows to the chocolate remaining in the pan. Mix thoroughly. Turn out into a 9" square pan. Pat out flat. Drizzle reserved chocolate over the top.
Refrigerate several hours or overnight. Dust top with powdered sugar and cut into 24 bars.