Place pizza dough on a floured surface and cut in two equal portions. Cover with a cloth and let rise for about 2 hours. Meanwhile, make the filling. In a large skillet, heat butter and add mushrooms. Cook until mushrooms are browned. Remove and set aside. In the same skillet, heat olive oil and sauté onion, leek and garlic until soft and translucent. Add in the mushrooms, stirring to combine. Add beef and cook until browned. Add ¼ cup pizza sauce, stirring to combine. Simmer for 30 minutes, stirring occasionally. When done, set aside to cool. In a bowl, combine ricotta and shredded mozzarella. Set aside. Using a rolling pin or by hand, roll out first pizza dough into an approximately 8 inch rounds. Spread pizza sauce over entire round, leaving 1/2 inch border. On the bottom half of the dough, place 4 slices of pepperoni. Spread half of the cheese mixture on top of the pepperoni, and then add 4 to 6 tablespoons of the filling on top of the cheese. Sprinkle basil over top. With a pastry brush, brush the outer edges of the dough. Carefully fold half of the dough over the half that has the filling. Press edges together to seal. Brush entire calzone with milk. Sprinkle lightly with salt. Repeat with second pizza dough. Bake at 350 degrees for about 25-30 minutes or until golden brown. Serve immediately with a side of the remainder pizza sauce.