Mix together the flour, the brown sugar, and the salt in a large bowl. Create a well in the center of the bowl and add the yeast, remaining teaspoon of sugar and ½ cup warm water. Let it stand until nice and foamy, about 5 minutes. 

Gradually mix in the remaining warm water. Dump the sticky mess onto a floured surface and knead the dough until smooth and elastic (about 5-10 minutes by hand, 3-5 minutes if using a stand mixer). Shape the dough into a large ball and place in a bowl slicked with canola oil. Cover with a damp cloth and place in a warm place.  Let rise until the dough has doubled in size, 45-60 minutes. 

Meanwhile, liberally grease the inside of a 3 quart Dutch oven (or an ovenproof saucepan with straight walls, a flat bottom, and a tight-fitting lid) with the butter (4 tablespoons). Sprinkle 1 teaspoon of the nigella seeds on the bottom of the pan. Divide the risen dough into six equal pieces. Flatten each piece with your fingers and smear 1 teaspoon of butter over the surface. Bring the edges up to enclose the filling and smooth it back into a ball. Spread a little more butter on the outside of the ball. Place one ball in the center of the pot and the rest around it like petals on a flower. Cover with a damp cloth and let it rise again for 20-30 minutes. 

Preheat oven to 215 degrees. Place the cubes of butter all over the top of the dough and sprinkle with the remaining nigella seeds. Cover tightly with a lid and cook overnight, start checking at 8 hours for a dark golden brown bread, if the bread looks undercooked, continue baking for another hour or so.