Savory Babka with Gruyère, Mozzarella, and Black Sesame

Directions

1. To make the dough, heat the milk until just lukewarm. Stir in the yeast and let sit for 5-10 minutes.
2. In the bowl of a stand mixer fitted with a dough hook, or by hand in a large bowl, mix together the milk/yeast mixture, flour, butter, sugar, egg yolk, salt, and cheese and garlic powders (if using). Mix until the dough comes together, and then knead until the dough is very smooth and elastic—don't skimp on this step. It should take about 10 minutes in a stand mixer. If the dough is still pretty sticky, carry on kneading until it feels quite smooth.
3. Lightly grease a large bowl and place the dough in it. Cover the bowl with plastic wrap or a damp tea towel and let rise for about 1 1/2 to 2 hours, until puffy and almost doubled.
4. Preheat the oven to 350°F.
5. Gently press down the dough to deflate it, and turn it out onto a counter. The dough is buttery enough that you shouldn't need extra flour—it shouldn't stick.
6. Press/roll/stretch the dough out into a large rectangle, about 12" x 18" in size.
7. Brush a thin layer of melted butter evenly over the dough, leaving a little space around all the edges (about 1/2"). Spread the mustard thinly over the butter.
8. Sprinkle the cheeses, scallions, and sesame seeds in an even layer over the dough.
9. Starting with the long side closest to you, roll the dough into a long log and pinch the seam firmly closed.
10. Using a sharp serrated knife, cut the log in half lengthwise. You’ll end up with two strips of dough. Take those two pieces and pinch two ends together. Twist the logs around each other two or three times; pinch together the other two ends. Place the braid, cut side up, in a greased 9″ x 5″ loaf pan.
11. Cover with plastic wrap and let rise for 20 minutes.
12. Bake for 30 minutes, then tent the top with foil if the cheese is browning too quickly, and bake for another 20 minutes. Remove from the oven and let cool for 10 minutes in the pan before turning out to cool on a wire rack.