Mix the sugar, vanilla sugar and butter with the whisk of the hand mixer on the highest setting until a solid mass is formed. Stir in eggs one at a time for about 1/2 minute.
Mix the flour, cornstarch and baking powder and stir in alternately with the egg liqueur on a low level.
Mix the plum jam and chopped almonds. Pour a third of the dough into a greased loaf pan, sprinkled with the sliced almonds, and smooth it out. Spread half of the plum jam on the dough (but not to the edge, otherwise the cake will stick in the pan later). Spread another third of the batter on top. Spread the rest of the plum jam on top and finally spread the last third of the batter.
Bake the cake in the preheated oven at 180 degrees (or 160 degrees convection) for about 65 minutes on the middle rack. Then let it cool down a little in the mold, then fall out of the mold and let it cool down. Dust with powdered sugar or cover with a glaze as desired.
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