Put the oranges in a pot and cover with cold water. Bring to the boil, turn down the heat and simmer for 2 hours. Top up with more water if they start to boil.
Preheat the oven to 170 degrees C. Line a springform 20cm cake tin with baking paper.
Remove the oranges from the water. Using a sharp knife, carefully remove the skin. Cut orange in half discarding any seeds. Place in the food processor with the caster sugar, eggs, ground almonds, almond essence and baking powder. Blitz until smooth.
Transfer the mixture into the prepared cake tin & sprinkle the slivered almonds over the top.
Bake in the pre-heated oven for 1 hour 30 minutes. Check it half way through and cover it with foil if it starts to burn on top.
When cooked let it cool in the tin for 10 minutes then remove from the tin and put it on the cake rack to cool completely.
For the orange cream, combine the yoghurt with the sugar, orange zest and Cointreau.
To serve, dust the plate with some icing sugar and serve the cake with a generous spoonful of orange cream.