Place the flour, sugar, dry yeast, salt, and baking powder in the bowl of a stand mixer (or mixing bowl if kneading by hand) fitted with the dough hook attachment. Mix until combined.
Add the warm almond milk, applesauce, and vanilla extract. Knead for about 2 minutes on slow/medium speed, or until the dough is smooth. Gradually add the butter and keep kneading until the butter is fully incorporated, about 3-5 minutes.
Transfer to a large bowl, cover with plastic film to touch to prevent the dough from drying, and let it rise in a warm place for about 2 hours, or until it has doubled in size.
Punch down to deflate the dough. Cover with plastic film again and refrigerate overnight, or for at least 8 hours to let flavors develop.
In the meantime, prepare the black sesame filling: Add the black and white sesame seeds to a skillet. Heat over medium heat, stirring regularly until white sesame seeds turn golden brown. Transfer to a bowl and let it cool.
Add the toasted sesame seeds and sugar to a blender. Blend until you get a fine powder. Set aside.
The next day, remove the dough from the refrigerator. Place it on a silpat or non-stick surface and roll it out into a rectangle of about 12x7-inch. Fold it in half width wise. Turn your rectangle of dough a quarter turn (90°) and roll it out again. Fold again. Repeat the same step once again and finally roll it out into a 12x7-inch rectangle. This will create a flakier and more airy texture.
Spread the black sesame sugar over the dough and slightly press with your hands to make it stick to it. If using black sesame paste*, simply spread it over the dough.
Tightly roll the dough out into a long log. Carefully transfer the log into a loaf pan (I used a 12x5-inch pan). Let it rise in a warm place for about 2 and 1/2 hours. The brioche will almost double in size.
Preheat oven to 350°F (175°C). Brush the top of the brioche with almond milk and bake for about 25 minutes, or until the top is golden brown. Remove from the oven and let it cool completely before removing from the pan and enjoying!
This brioche will keep for up to 3 days at room temperature, wrapped in plastic film.
Notes
*If you have black sesame paste on hand and want to save some time, feel free to mix 2 tbsp of paste with 2 tbsp of sugar.