Cook the oranges in a saucepan covered completely with cold water for 30 minutes. When cool, chop the oranges and remove seeds. Blend in a food processor until smooth. Set aside.
Heat the oven to 180ºC.
Cream butter and brown sugar, until syrupy. Spoon the butter mixture into 12 muffin tins and put slices of oranges in the bottom.
Beat the eggs and sugar until fluffy and thick. Fold in orange mixture, baking powder and ground almonds, stirring well until well blended. Spoon into the muffin tins.
Bake for 30 minutes, or until set. Let it cool in the tin for 15 minutes.
When done, quickly invert the muffin tin and lift off gently. Pick up any orange slices that may have remained in the tin and place on top of the appropriate muffin.
