Preheat oven to 180 degrees celsius (fan forced). Grease and line a 20cm x 30cm baking tray with baking paper and set aside.
Cream the butter and sugar on Speed 4 until pale and creamy (scraping down the sides occasionally).
Add the sifted plain flour, baking powder and ground ginger and mix for 6 seconds on Speed 6. Knead for 1 ½ minutes on Interval Speed.
Press the dough firmly into the prepared tin and smooth the top with a spoon.
Bake for 20-25 minutes or until lightly golden.
Place the butter, icing sugar, golden syrup and ground ginger into the Thermomix bowl and melt for 3 minutes on Speed 3, 80 degrees, or until melted.
Pour the icing over the warm slice.
When the icing has just set, cut into slices.