Preheat oven to 180˚C. Wash the oranges. Place into a saucepan. Cover with water and simmer for 1 hour. Drain. Cool. Puree in a food processor until smooth.
In a large bowl cream the butter and sugar until light and fluffy. Add the eggs one at a time. Fold in the flour, ground almonds, slivered almonds, baking soda, buttermilk, vanilla and finally the orange puree. Mix gently. Spoon the mixture into a 26cm greased ring tin and bake at 180˚C for 45 minutes, or until the cake is golden brown and bounces back when pressed.
While the cake is baking heat the sugar, orange juice, water and orange zest in a small saucepan. Bring to the boil and simmer gently for 10 minutes. Drizzle syrup over the warm cake. Remove the cake from the tin when it’s cool.