1. Grease a 10" springform pan, and preheat oven to 375 degrees.
2. Plum layer: wash plums and dry. Slice in half and remove pits. Slice into pieces approx 1-2 mm wide. Lay flat in a single layer on a paper towel to dehydrate slightly while the rest of the cake is prepared.
3. Dough layer: In a large bowl, soften butter in microwave. Add marzipan and mix until mostly well-blended. Add sugar and lemon zest and mix until well-blended. Add eggs one at a time, mixing well after each egg. In a separate bowl, add baking powder to flour. Then, flour/baking powder mixture to the wet ingredients, a little at a time, while mixing well. Once dough is well mixed, set aside.
4. Bottom layer: mix almonds and sour cream in a saute pan; gently saute over medium heat being very careful not to overcook the almonds. Once lightly toasted, remove from heat, add a pinch of salt, and crush the almonds manually into smaller pieces. Distribute evenly across the bottom of pan.
5. Streusel layer: In a bowl, melt butter, add vanilla, and sugar. Mix well. Then blend in flour by hand. Mix by hand to make crumbs.
6. Using a large spoon, gently place dough over almonds, and spread dough evenly to the edges of the pan, while being careful not to displace the almonds (hint: use your spoon to push the top of the dough from the center of the pan out towards the edges). Next, arrange the plums over the dough. Press the plums down into the dough a bit after each layer, except the top one. You will likely have 3-4 layers of plums to place. Use your hands to make crumbs of the streusel mix, and sprinkle the crumbs evenly over the cake. Bake on center rack. Check for doneness at about 48 minutes. May take 10 minute longer if plums are very juicy. Do not over-cook the streusel layer. Cool. Serve w/whipped cream.