In a large bowl, mix together the flour, yeast, sugar and salt. Set aside. In a small saucepan, melt the butter, then remove it from the heat and whisk in the cold milk. Add the egg and whisk to combine. Test the temperature of the liquid with your finger, it should be slightly warm but not hot. Let cool until warm if needed. Add the liquid to the dry ingredients and mix with a wooden spoon until the dough starts to come together. Transfer it to working surface and knead gently until dough is smooth and elastic. Cover and let rise until double in volume (approximately 1 hour at 22 degrees room temperature).
In another large bowl, pit the plums, slice them each into quarters lengthwise and set aside.
Grease the bottom and sides of a large baking sheet with butter and then lightly dust the bottom of the pan with sifted flour. Move the pan around to cover all the butter with flour and then shake the pan over the sink to remove any excess flour. Set aside.
Flour a work surface and roll the dough out evenly to the approximate size of the baking sheet. Fold the dough in half and gently slide it onto the pan. Gently press the dough into the corners of the pan if needed. Arrange the plums skin side down and side by side to fill the entire pan. Allow the dough to rise a second time for 30 minutes.
Preheat the oven to 375 degrees F. Bake for 30 minutes or until the dough is golden brown. Remove from the oven, mix the sugar and cinnamon together in a small bowl and sprinkle evenly over the plums then sprinkle the sliced almonds on top. Broil for 2 minutes until the almonds are toasted. Remove and let cool slightly before serving. Best served with a dollop of whipped cream!
1. Place a sheet of aluminum foil on the oven rack below the baking sheet to help keep the dough from burning (only needed if you have a crazy electric oven like I do).