Preheat the oven to 200c/180c Fan/Gas 6. Grease a 9 inch circular cake tin (I use one with a pop out base for extra ease in removing).
In a large bowl, or the bowl of a stand mixer, combine the melted butter and sugar until fully combined (about 3 minutes). Add in the egg, lemon zest and lemon juice and mix again.
Fold in the flour and baking powder to form a loose cake batter. Pour this into the tin and use a spatula to ensure an even layer. Set aside.
Make the topping: combine all ingredients apart from the blueberries in a large bowl. This should form a creamy, runny mixture. Pour this over the cake gently, spreading to the edges. Scatter over the blueberries.
Bake for 40-45 minutes, checking on it at about 30 minutes to ensure it isn't getting burnt at the edges. If it gets too golden/dark, cover with tin foil for remaining bake time. The result should only have a very small 'wobble' when it is ready!
Remove from the oven and let cool on a wire rack. Dust with icing sugar and serve!
