Cream butter with sugar and vanilla sugar with the whisk of the hand mixer. Stir in the eggs one after the other. Mix flour and baking powder, stir in sour cream briefly. Fill the dough into a greased, square springform pan (24 x 24 cm) and smooth it down.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 50-60 minutes (stick test!). Let cool down on a cake rack.
Chop the couverture, melt with coconut oil over a warm water bath. Remove the cake from the mould. Cut in half horizontally with a knife. Spread the lower bottom with jam, place the upper bottom on top.
Cut the cake into approx. 34 cubes (each 3.5 cm long). Straighten the edges of the cake a bit. Place the cubes on a cake rack and cover with the chocolate coating. When the chocolate coating begins to set, place the chocolate lentils on top of the cubes as cube eyes.
Allow to dry.