Beat softened butter in a large bowl until fluffy. Add sugar and beat til blended. Beat in egg, citrus peels, salt, and mahleb. Beat in milk, then yeast. Don’t worry if the mixture seems to “break” or be a little less smooth; it will come back together as you add the flour. Beat in 1 1/2 cups flour 1/2 cup at a time. Add remaining flour, about 1 cup, using a spatula. The dough should be soft and slightly sticky. Knead in the bowl, then turn onto lightly floured surface. Knead about 5 minutes; you want a nice soft, satiny dough in the end; add small amounts of flour if needed.