Preheat the oven to 180 C / Gas 4. Grease and line a 20cm spring form cake tin.
Place the oranges in a saucepan, cover with cold water, bring to the boil and cook for 15 minutes. Drain and repeat. Coarsely chop and remove seeds.
Place the chopped oranges in a blender with a splash of cold water and pulse until smooth.
In a separate bowl, beat the eggs and sugar with an electric beater until thick and pale. Add the orange, ground almonds, flour and baking powder and fold to combine.
Pour the mixture into the cake tin and bake for 1 hour or until a skewer inserted into the centre comes out clean. Set aside for 15 minutes to cool while you make the syrup.
Finely grate the rind of the whole orange, then juice. Boil the rind for 5 minutes in a saucepan to soften, then drain and return to the pan. Add the orange juice and sugar and heat on low stirring constantly for about 3 minutes until the sugar dissolves and it becomes syrupy.
Gently poke holes all over the top of the cake with a skewer and then spoon over the syrup. Serve warm or at room temperature.