Prepare the Baking Pan: Preheat the oven to 375°F (190°C). Grease a 9-inch springform pan and lightly coat it with flour, shaking off any excess.
Grind the Nuts: Place the toasted almonds and hazelnuts in a food processor. Process until they resemble fine crumbs. Stop before the nuts become oily or turn into nut butter.
Cream the Butter: Add the room-temperature butter to a large mixing bowl. Beat with an electric mixer until smooth and creamy.
Incorporate the Sugar: Gradually add the confectioners’ sugar while mixing on low speed. Continue until the sugar is evenly combined, scraping down the sides of the bowl as needed.
Mix the Cake Batter: Add the eggs, all-purpose flour, and baking powder to the butter mixture. Mix until the ingredients are incorporated into a thick batter.
Fold in the Ground Nuts: Use a wooden spoon or rubber spatula to gently fold the ground almonds and hazelnuts into the batter until evenly distributed.
Transfer the Batter: Spoon the thick batter into the prepared springform pan. Spread it evenly across the pan to create a level base for the fruit.
Arrange the Plums: Place the prepared plum slices over the batter in a circular pattern. Arrange them in a single layer without overlapping.
Begin Baking: Place the pan on the middle rack of the preheated oven. Bake at 375°F (190°C) for 15 minutes.
Reduce the Oven Temperature: After 15 minutes, lower the oven temperature to 300°F (150°C) without removing the torte from the oven.
Finish Baking: Continue baking for another 30–40 minutes. Check the center by inserting a toothpick; the torte is ready when the toothpick comes out clean.
Warm the Apricot Preserves: Place the apricot preserves in a microwave-safe bowl. Heat for approximately 30 seconds, or until the preserves become runny enough to brush over the cake.
Glaze the Torte: Once the torte has finished baking, brush the warm apricot preserves over the top to give the plums a glossy finish.
Cool and Serve: Allow the torte to cool slightly before slicing. Serve warm for a comforting dessert, or let it cool further before serving.