Melt the dark chocolate over a double boiler. Once melted, transfer to a small bowl and stir in the hazelnut butter, powdered sugar, coconut oil, and vanilla extract. Stir until fully combined. Cover and refrigerate for about 2 hours, or until it has just slightly thickened, you don't want it to harden too much.
Dough
Add the bread flour, warm almond milk, sugar, salt, baking powder, and vanilla to a stand-mixer. Knead for 10 seconds on low speed. Next, add the dry yeast and knead on medium speed for about 30 seconds.
Add the vegan butter and knead on medium-high speed for 3-5 minutes, or until the butter is fully incorporated and the dough is smooth.
Transfer to a bowl and cover with plastic film to touch (or a wet towel) to prevent the dough from drying. Let it rise for about 2 hours in a warm place, or until it has doubled.
Punch the dough to deflate it, replace the plastic film to touch, and refrigerate for at least 2 hours, or overnight.
After 2 hours, roll out the dough into a 13x10-inch rectangle between two sheets of parchment paper. Spread the chocolate hazelnut filling evenly on the rectangle of dough. Note: if the chocolate hazelnut filling is too thick/hard, place it in the oven with the light on for 30 minutes, or until it has softened.
Roll up the dough tightly to make a 10-inch long log. Cut the roll into 8-9 rolls and place them on a baking sheet lined with parchment paper, or in a large baking dish. Make sure you leave at least 4 inches between each, as they will double in size.
Cover with plastic film and let it rise for another 2 hours at room temperature. The rolls/buns will almost double in size.
Preheat oven to 350°F (175°C). Slightly brush each bun with some almond milk. Bake the rolls for 20-25 minutes, or until just golden brown. Remove from the oven and let it cool 10 minutes before enjoying!
These rolls are best served warm but will keep for up to 3 days at room temperature. You can reheat them covered with foil in a 350°F pre-heated oven for 15-20 minutes.
Notes
If you end up having too much chocolate hazelnut filling, melt it over a double boiler and drizzle over the buns!