In the bowl of a stand mixer, add active dry yeast, lukewarm milk and 1 tbsp. sugar. Give a quick stir and let the yeast proof for about 15 minutes.
In a medium bowl, add flour, remaining 2 tbsp. sugar and salt; whisk until blended.
When yeast is proofed, add egg and lemon zest. Using the paddle attachment, process on medium-high until mixed.
Add dry ingredients, melted butter and pure vanilla extract. Switching the attachment for dough hook, process on speed 2 for 2 minutes.
Increase to speed 3 and process until dough clings around the hook, add a tbsp. of remaining ¼ cup flour at a time.
With oiled hands, take the dough and pull under to form a ball (it will be very sticky). Place in a lightly greased bowl and toss around to coat. Cover with a clean dish towel and transfer to a draft-free area such as the oven (with heat off). Let it rise for 1 to 1:30 hours or until it doubles in size.
Meanwhile make the streusel.
Remove the dough from its spot and drop it onto a lightly floured work surface (don’t punch the dough). Stretch it to create a rectangle of about 9 x 13.
Place the dough in a cake pan that has been lined with a greased parchment paper with overhangs. Stretch it to cover the bottom of the pan.
Lay plum slices tightly together in a single layer, lining them up in rows.
Cover with a clean dish towel and transfer to its spot to proof for 30 minutes.
Half an hour later, remove from its spot and preheat oven to 375ºF.
Sprinkle on streusel evenly on top of plums, making sure to cover them all.
Transfer the cake to the preheated oven and bake for 26 to 30 minutes or until a cake tester inserted in the center comes out clean.
Turn the heat to Low Broil and cook for 5 minutes or until the top is golden brown.
Remove from the heat to a wire rack and allow to cool for 15 minutes before carefully lifting the cake from the pan, back to the wire rack.