Measure 1 cup of warm milk. Stir 1½ tablespoons of active dry yeast into three-quarters of it and let stand until foamy.
Mix in 4 cups of all-purpose flour, ¾ cup of sugar, ½ teaspoon of salt, ½ cup of melted butter, 2 eggs, and 1 teaspoon of vanilla extract. Knead for 4-5 minutes, adding the remaining milk as needed.
Cover and let rise in a warm place for about 1 hour, until doubled.
Preheat the oven to 350°F / 175°C. Press the dough into a buttered rimmed baking sheet at least 18in / 46cm long. Arrange 1360g of pitted, quartered Italian plums close together in rows on top.
For the streusel, mix 1 cup of all-purpose flour, 1 cup of sugar, 1 teaspoon of ground cinnamon, and ½ cup of softened butter until crumbly. Scatter over the plums.
Bake for 30-35 minutes, until golden. Let rest for at least 10 minutes before cutting into squares.
