Preheat the oven to 160℃ fan oven. Spray a cake tin with baking spray or line it with parchment paper.
Cream together the butter spread and sugar until light and fluffy. This usually takes about 7 minutes. Add the eggs, one at a time, beating well after each addition.
Add the flours, baking soda and cardamom together over the mixture, add the vanilla seeds, zest of 2 limes and fold in gently, using a spatula, until just combined. Lastly, fold in the grated courgette. Spoon the cake mixture into the prepared tin and gently level the surface. Bake on the middle shelf of the oven for 50-60 minutes. To test, insert a skewer into the middle of the cake: it should come out clean; if not give it a little longer.
Once cooked, leave the cake to cool in the tin for 5 minutes, then transfer to a wire rack.
To make the icing, in a bowl, whisk the icing sugar, ricotta and lime juice together until smoothly combined.
Spread the icing on top of the cake and sprinkle with the zest of one more lime. Cut into 14 slices to serve.
If you are making the secret filling: combine the condensed milk and the lime juice together. Using a syringe pump the filling into the warm cake mixture before you put the icing on top. I highly recommend you don’t skip this step. It really does add such an element of surprise and deliciousness to the cake.