Preheat oven to 180°C/350°F/Gas 4. Line a 28cm x 18cm/12" x 7" baking tin with parchment paper.
Cream the butter and sugar together until smooth. Add the eggs and beat briefly.
Sieve in the flour and baking powder, then stir gently until combined.
Beat in the milk for a minute until the mixture becomes a thick batter. Stir in the lemon rind.
Pour into the prepared tin and smooth the top, making a very slight dip in the centre to get a flat top as it bakes.
Bake for 35-45 minutes until the cake is golden brown, springs back when pressed and is shrinking away from sides of the tin.
Leave to cool slightly in the tin, then transfer to a wire rack to cool completely.
Sieve the icing sugar into a bowl, then whisk in the lemon juice to make a thick paste. Spread evenly on top of the cooled cake and smooth with a knife or the back of a spoon. Decorate with grated lemon rind on top and slice into squares to serve.