Preheat the oven to 350°F (180°C). Grease a 9x9-inch pan, and line with two pieces of parchment paper, so that there is an overhang 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, and mix well to combine.
In a small pot, melt together the butter, golden syrup, and vanilla. Add the wet ingredients to the dry ingredients, and mix well. Press the mixture into the tin. Bake for 25-30 minutes, until lightly golden on the edges. Five minutes before the base is done, prepare the topping.
To Make the Topping:
Add all of the ingredients to a medium pot in the order listed. Heat over low heat, stirring constantly, until smooth. Pour over the hot base, and leave 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.