Soften yeast in the lukewarm water.
Stir in milk.
Blend butter with flour and stir in eggs, sugar, salt, and masticha..
Add yeast mixture and blend thoroughly.
Turn out on lightly floured board and knead for 10 minutes or until dough is smooth and elastic.
Put into greased bowl, cover, and let rise in a warm place for about 2 hours or until double in bulk.
Punch down dough and knead again until smooth.
Divide in half and form each half into a loaf.
Place loaves in large floured pans and make 5 depressions in each, 1 in the center and the others at the 4 opposite edges of the dough.
Place eggs in these depressions.
Cover pans and let dough rise for about 1 1/2 hours or until double in bulk.
Brush tops with beaten egg and sprinkle with sesame seeds.
Bake in a 350 degree oven for 45 minutes or until bread is golden.