Pre-heat the oven to 180°C and line a 20cm square baking tin with baking paper.
In a medium sized pot, melt the butter and sugar together. Remove from the heat and beat in the egg and vanilla.
Stir in the flour, baking powder and salt until the dough comes together.
Place ¾ of the dough in the lined baking tin and reserve the rest, wrap it in gladwrap and place in the freezer. Press the dough in the tin so that it is evenly spread to the edges.
Now prepare the filling. Melt all filling ingredients together over a medium heat in a small pot on the stove. Pour over the unbaked base.
Remove the ¼ dough from the freezer and using a cheese grater or your hands, roughly grate or crumble the dough over the filling.
Bake for 30-35 minutes until the filling is bubbling and is starting to go golden brown around the edges.
Remove from the oven and cool completely in the tin before removing and slicing.
Store in an airtight container at room temperature for 10 days or freeze.