Sift the flour together with the baking powder and powdered sugar. Add salt and pieces of cold butter. Chop with a knife or pastry cutter or combine in a food processor quickly to make the shortcrust pastry. Add the egg yolks and knead all to a uniform dough. You can use hand mixer with hook attachment to prepare the dough. Divide the finished dough into two parts (one a bit larger than the other - 2:3 , the larger part will be the bottom), wrap it with cling film, and chill for 2 hours.
Line a square baking pan with dimensions 9x13 inches / 25 × 35 cm with baking paper. Grate the bigger half of the dough into the baking pan using a box grater and spread it evenly, pressing it lightly to form the base. THis sounds unusual but it works! We bake this cake base at 180 C - 365 for about 20 minutes. After the baking time, take it out of the oven and sprinkle with half the poppyseeds or breadcrumbs and nuts. Arrange the plum halves with the cut side up on top of the batter and sprinkle with the remaining nuts and poppy seeds.