I a small sauce pot place both butters, milk, sugar and mahlep. Heat over low heat until butters are melted. Do NOT boil.
In a small bowl dissolve the wet yeast in the luck-warm water. Pour in the small sauce pot.
In the previous bowl crack and whip the eggs. Pour all wet ingredients in the bowl of a stand mixer. Attach the dough hook and stir to combine.
With the mixer running on low speed add the flour cup by cup until well incorporated. Knead the dough for 7 min. Place the mixer bowl in the oven. The oven should be OFF. Let the dough rise until double in size. (30-60 min).
Punch the dough down and divide it in 6 equal parts. Roll out 6 strands and form two 3-strand braids. Cover them and let them rise in a draft free space until double in size.
Preheat oven in 180C (360F). Before baking, thoroughly egg-wash the loaves. If desired sprinkle them with slivered almonds.
Bake until brown. ~20 min.