Adapted from The Edible Garden.
90g caster sugar
225g plain flour
1 egg yolk
1 cup pure icing sugar
1. Preheat oven to 160 degrees celcius. Line 2 baking trays with baking paper.
2. Cream the butter and sugar together in a bowl until light and fluffy.
3. Add the flour & egg yolk and mix well until combined.
4. Roll mixture into a ball and place on a floured surface. Knead gently.
5. Using a rolling pin, roll the mixture out until it is approximately 1cm
6. Using an appropriately sized cutter, cut circles in the dough and gently place them on the baking tray, being careful to leave a little room in between as they will spread a small amount.
7. Using a toothpick, make 4 holes in the centre of the biscuits. Place into the oven and bake for 15-20 minutes or until the biscuits are very slightly golden on top.
8. Allow to cool on the trays for a few minutes before carefully transferring to a cooling rack.
10. To make the icing, place the icing sugar and mulberries in a bowl. Start with 4 mulberries and wait to see if you need the 5th. Using electric beaters or a mixer, mix on high until combined.
11. Wait until biscuits are completely cooled before spreading the icing on one side and sandwiching with another biscuit.