Place oranges in a heavy based saucepan (with a lid). Add enough water to cover the oranges and bring to the boil. Simmer for around 2 hours. Remove oranges from water and set aside to cool.
Pre-heat oven at 180degC/350degF.
Grease and line a 10cm deep 23cm round cake tin.
Chop up oranges and remove seeds. Place oranges in a food processor and blitz until smooth.
Melt chocolate, either in microwave or over a double boiler, use your preferred method.
In a large bowl mix orange puree, eggs, sugar and baking powder. Mix in melted dark chocolate, then fold through almond meal and ground almonds.
Bake for approximately 1 hour or until firm to touch and when tested with a skewer it comes out clean.
Remove from tin and cool on a cake rack.
While cake is cooling make the syrupy orange slices. In a medium heavy based saucepan place water and sugar. Over medium heat, stir until sugar has dissolved. Add orange sliced and bring to a simmer. Simmer for around 20 minutes until orange slices are tender and liquid has become a thick syrup.
Top cake with orange slices and some of the syrup before serving.