Combine warm water (110°F), yeast, and 1 tsp sugar in a large bowl; let sit 5 minutes until foamy. Whisk in the warmed milk and 1 cup flour until smooth, then cover and proof for 45–60 minutes until doubled and spongy.
Whisk together 6 cups flour, sugar, and salt in a large bowl, then rub in the orange zest with your fingertips to release the oils. Add mahlepi and mastic if using, then set aside.
Whisk together the cooled melted butter, 4 whole eggs, 1 egg yolk, and vanilla extract in a medium bowl until light and slightly frothy, about 1 minute.
Transfer the proofed yeast mixture to the stand mixer bowl, add the wet ingredients, and mix on low. Gradually add the flour mixture one cup at a time until the dough pulls away from the sides and is soft and slightly sticky.
Increase speed to medium and knead for 8–10 minutes (12–15 by hand) until the dough is smooth, elastic, and springs back when poked. Add flour sparingly only if the dough is excessively sticky.
Shape the dough into a ball, place in a lightly oiled bowl, turn to coat, and cover tightly with plastic wrap and a kitchen towel. Let rise in a warm spot for 1.5–2 hours until doubled in size.
Punch down the dough, knead briefly, then shape into a smooth round and place in a generously buttered 10-inch pan. Make a small incision in the bottom, insert the foil-wrapped coin, and press the dough to seal.
Cover loosely and let rise 45–60 minutes until puffy; preheat oven to 350°F (175°C) during the last 20 minutes. Brush with egg wash, sprinkle with sesame seeds, and decorate with almonds if desired.
Bake on the center rack for 45–55 minutes, rotating halfway, until deep golden brown and the internal temperature reaches 190–195°F (88–91°C). Tent with foil if the top browns too quickly after 30 minutes.
Cool in the pan 10 minutes, then transfer to a wire rack and immediately brush with the combined melted butter and honey. Allow to cool completely, at least 1 hour, before slicing.