1.Make quick plum jam: Combine all the ingredients in a small saucepan with ¼ cup (60ml) water; bring to the boil. Reduce heat; simmer uncovered for 20 minutes or until a spoonful of the jam wrinkles when pushed with a finger on a chilled plate. Cool.
2.Preheat oven to 180°C (160°C fan-forced) Line three oven trays with baking paper.
3.Beat the butter and sugar in a bowl with an electric mixer until pale; add egg, beat until just combined. Stir in sifted flours and cinnamon. Refrigerate dough for 20 minutes.
4.Roll 1 1/2 teaspoons of the chilled dough into a ball; place on tray. Repeat with remaining dough. Press a deep indent into the centre of each ball; fill with the plum jam. Bake biscuits for 12 minutes or until golden brown. Cool on a wire rack.
5.Dust with sifted icing sugar, if desired.
Jam drops will keep in an airtight container for up to two days. Suitable to freeze. Not suitable to microwave.