Wash oranges hot, dab dry, grate peel, squeeze juice and put aside. Put flour, sugar, eggs, buttermilk, salt, baking powder and orange zest into a bowl and mix with the whisk of the hand mixer to a smooth dough.
Grease a box mould (22 cm long) and sprinkle with breadcrumbs. Pour in the dough and bake in a preheated oven (electric: 175 °C/ gas: level 2) for 60 minutes. Melt butter.
Prick the cake from above with a wooden stick and sprinkle with 50 ml orange juice and butter. Let the cake cool down. Heat orange marmalade and almond slivers in a saucepan and spread evenly on top of the cake.
Serve with whipped cream. Makes about 20 pieces.