Preaheat oven to 350. Grease a 9"x9" tin and line with twopieces of parchment paper so that there is paper sticking out of the tin on all sides, forming a "sling".
In a large bowl, sift together the flour, baking powder, salt, and ground ginger. Add the brown sugar and rolled oats. Mix well to combine.
In a small pot, melt together the butter, treacle and vanilla. Add to the dry ingredients and mix well. Press the mixture into the tin and bake for 25-30 minutes, until lightly browned on the edges. Five minutes before the base is done, prepare the topping.
For the topping: add all the Topping ingredients to a medium pot in the order listed. Heat over low heat, stirring constantly, until smooth. Pour over the hot base and leave to set at room temperature until set.
Slice into pieces with a hot knife. Store in an airtight container. Can be kept in the fridge but bring to room temperature before eating.