Warm two cups of milk and place in a large bowl. Add the yeast, one cup of the flour, and 1/4 cup of the sugar. Cover and proof for one hour.
In a large bowl, combine seven cups of flour, the ground almonds, salt, remaining sugar, orange rind, aniseed or mahlepi and masticha (if using). Make a well in the center. Add the yeast mixture, melted butter and eggs. Work from the center outwards, bringing flour into the well, stirring the mixture until a dough begins to form.
Dust a worksurface with a little of the remaining flour and knead, adding more flour if necessary, until the dough is smooth and doesn't stick to your hands, about 12 minutes.
Place in an oiled bowl, cover with a cloth, and set aside in a warm, draft-free place to rise until doubled in bulk, about two hours. Punch down dough.
Divide into six small balls and roll each into strips 12-15 inches long, and about 2 inches in diameter. Lay three strips side by side, pinching together at one end, and braid. Pinch together at the other end to hold the loaf intact.
Optional: At this point you can press two red-dyed eggs between the strips of the braid or just leave the braided loaf plain.
Repeat the procedure to make the second loaf.
Place the breads on a parchment-lined baking sheet, covered, and let rise for two hours, or until doubled in bulk. While the braids are rising, preheat oven to 360F (180°C), placing rack on lower shelf of oven.
Beat together the egg yolk and remaining milk. Brush over tsoureki loaves and sprinkle with slivered almonds.
Bake for about 40-45 minutes, or until golden brown. I check my breads after about 15 minutes as they do tend to colour quickly and drape them with aluminum foil to prevent excess browning. The bread should sound hollow when tapped on the bottom. Remove and cool on racks.
*The special red dye used by Greeks to dye Easter eggs, as well as the spices Mahlepi (Mahleb) and Masticha can be found at all good ethnic grocery stores and in the Penzeys Catalague.