Preheat the oven to fan 160C/conventional 180C/gas 4. Lightly butter 12 muffin moulds and line each base with a disc of parchment paper.
Cream the butter and sugar together in a food processor or with an electric mixer. Add the eggs one at a time, beating each one in well, then mix in the orange zest and soured cream before folding in the flour (don’t worry if the mix looks curdled before adding the flour). Divide between the moulds and bake for 20-25 minutes or until firm to the touch.
While the cakes are baking, pour the orange juice and sugar into a pan, scrape in the vanilla seeds and add the pod, too. Bring to a rapid simmer and cook for about
15 minutes until thickened to a syrupy consistency. Remove the pod, replacing it with the papaya, and simmer gently for a few minutes until the papaya begins to soften.
Turn the cakes out of their moulds and peel off the paper, then turn them the right way up and serve topped with generous spoonfuls of syruped papaya and a light dusting of icing sugar. You can make the cakes and the syrup up to 24 hours ahead and keep them in separate airtight containers. Reheat gently just before serving, or serve cold.