To make the sponge, place the yeast in a medium bowl and add the milk and water. Whisk the mixture well then let it sit for 10 minutes. Add the flour, stir to combine, then cover the bowl with a towel or plastic wrap and let it proof in a warm spot for 45 minutes.
To make the dough, place the sponge in the bowl of a stand mixer. Using the paddle attachment, slowly add the eggs one at a time. Add the sugar, salt, and flour, then add the softened butter. When the dough comes together, place it in a buttered container and refrigerate overnight.
When you’re ready to use the dough, let it sit at room temperature for 1-1 ½ hours or until it’s slightly warmed. Meanwhile, preheat the oven to 325F.
Once it's room temperature, roll out and form the dough into either a ring or sheet tray. Press the plum slices into the dough and let it rise for 20 minutes. Sprinkle with sugar and bake for 20 minutes more or until golden brown.