Preheat the oven to 170°C/ Fan 150°C/Gas 3. Line a 21 cm round cake tin with baking paper. Whisk together the flour, baking powder and coconut in a large bowl and set aside.
In a stand mixer, cream the butter, sugar and lemon zest until light and fluffy, then add the eggs one at a time, beating well after each addition. Turn the mixer to low speed and mix in the ricotta, then add the flour mixture in three goes until just combined.
With the mixer still running drizzle in the milk and lemon juice. Turn off the mixer and fold in the rhubarb, then pour into the prepared cake tin.
Bake for 50 minutes or until lightly golden on top and a skewer inserted into the centre comes out clean. The cake will keep in an airtight container in the fridge for up to 1 week.