Warm the milk to 38 C / 100 F in a small saucepan. Stir in the yeast and 1 tsp of the sugar and let sit for 10 minutes until foamy.

In a large bowl or the bowl of a stand mixer, combine the flour, remaining sugar, vanilla sugar, and salt.

Add the yeast mixture, egg, and softened butter. Knead on medium speed for 8 minutes, or by hand for 12 minutes, until the dough is smooth, slightly tacky, and pulls away from the bowl.

Shape into a ball, cover with a clean kitchen towel, and proof in a warm spot for 45 to 60 minutes until visibly puffy and roughly doubled.

Combine flour, brown sugar, cinnamon, and salt in a medium bowl.

Add cold butter cubes and rub into the flour mixture with your fingertips until you have irregular clumps ranging from pea-sized to almond-sized. Do not overwork. Refrigerate until needed.

Halve each plum along the seam and remove the pit. Score the flesh side of each half with a shallow cut so it opens slightly like a fan.

Set aside on a paper-towel-lined tray while the dough finishes proofing.

Heat the oven to 180 C / 355 F. Grease a 9x13 inch baking pan with butter or line with parchment.

Punch down the dough and press it into the pan evenly using your palms and fingertips, building it slightly up the sides. Cover and let proof for a second time, 20 minutes, until slightly puffed.

Arrange plum halves flesh-side up in tight overlapping rows across the entire surface of the dough. Sprinkle with the 2 tbsp granulated sugar.

Scatter the cold streusel evenly over the plums, covering the fruit generously.

Bake on the middle rack for 38 to 42 minutes until the streusel is deep golden brown and the dough edges are set and pulling away from the pan.

Transfer the pan to a wire rack and cool for at least 20 minutes before cutting. Dust with powdered sugar before serving if desired.