Preheat the oven to 190°C/375°F/Gas mark 5. Grease a 20cm/8in springform cake tin.
Cream the butter with 150g/5oz of the sugar until light and fluffy. Sift together the flour, baking powder and salt. Add to the flour mixture alternately with the eggs. Beat in the lemon zest and juice, then the milk. Put the mixture into the prepared cake tin, and level the top.
Mix the rhubarb with the cinnamon and remaining sugar, and spread on top of the cake mixture. Bake in the oven for 40–45 minutes until the rhubarb is soft and the cake has shrunk slightly from the side of the tin. Leave to cool completely in the tin.