Preheat the oven to 350F/180C. Grease a 13 x 9-inch pan with butter and line the pan (up the sides too) with parchment and grease that with butter too.
Measure all the ingredients into a large bowl and beat for 2 minutes, or until well blended. Turn the mixture into the prepared pan and level the top.
Bake for 35–40 minutes, until the cake has shrunk a little from the sides of the pan and springs back when lightly touched with a fingertip in the centre of the cake.
To make the glaze. Mix the sugar with the lemon juice and stir to a runny consistency.
Leave the cake to cool for 5 minutes in the pan, then lift out, with the lining paper still attached, and place on wire rack set over a tray.
Brush the glaze all over the surface of the warm cake and leave to set. Remove the lining paper and cut into slices to serve