Preheat oven to 170°C. Brush a round 22cm (base measurement) springform pan with melted butter to lightly grease. Line base with non-stick baking paper.
Place the oranges in a saucepan and cover with cold water. Bring to the boil over medium heat. Cook for 15 minutes or until tender. Drain. Return to pan and cover with cold water. Bring to the boil and cook for 15 minutes (this will reduce the bitterness of the peel).
Refresh under cold water. Drain. Coarsely chop oranges. Remove and discard any seeds.
Place the orange in the bowl of a food processor and process until smooth.
Use an electric beater to whisk the eggs and sugar in a bowl until thick and pale. Add the orange, almond meal and baking powder and gently fold until just combined. Pour into prepared pan.
Bake for 1 hour or until a skewer inserted into the centre comes out clean. Set aside for 15 minutes to cool.
Meanwhile, to make the orange syrup, use a zester to remove the rind from the orange. (Alternatively, use a vegetable peeler to peel the rind from orange. Use a small sharp knife to remove white pith. Cut rind into thin strips.) Juice orange.
Place rind in a saucepan of boiling water and cook for 5 minutes or until soft. Drain.
Return to pan with orange juice and sugar. Place over low heat and cook, stirring, for 2-3 minutes or until the sugar dissolves and the syrup thickens.
Turn cake onto a serving plate. Use a skewer to gently prick the top. Spoon over syrup. Cut into wedges to serve.