Place mandarins in a large saucepan and cover with water. Bring to boil, reduce heat to medium and simmer 1 hour, until very soft. Drain. When cool enough to handle, cut in half and remove any pips. Gently squeeze mandarin halves to remove excess liquid but do not squeeze dry. Place mandarin halves in a food processor, with skin intact, and process to a puree.
Preheat oven to 200°C. Grease and line a 22cm springform pan.
Using an electric beater, beat eggs and sugar together until pale and thick. Fold in almonds and baking powder. Stir in mandarin puree and pour mixture into prepared pan.
Bake for 50 minutes, until a skewer inserted comes out clean. Cool completely in pan. Turn out onto a serving plate and dust with icing sugar. Serve wedges of cake with thick cream or sour cream.