Preheat the oven at 170°C.
In a large bowl, mix melted butter, sugar and grated zest. Add eggs, flour and baking powder and then lemon juice, stir until combined.
Line base and side of the pan with non stick baking paper. Spoon mixture into prepared pan and spread evenly. Bake for 30 to 40 minutes, until a thin bladed knife inserted into the centre of the cake comes out clean.
Sit pan on a wire rack, roll up the cake in a plastic film in order to keep it moist. Cool completly before icing .
Combine the icing ingredients.Drizzle over the cake.