Preheat the oven to 350 degrees Fahrenheit.
Add 1/2 cup cold water, 1/4 cup cocoa powder, 1/4 cup pure maple syrup, 1/4 cup creamy peanut butter, 1 teaspoon pure vanilla extract, and 1/4 teaspoon salt to a large mixing bowl.
Use a whisk to mix well.
Add 1 cup vital wheat gluten (spooned into the measuring cup then leveled--not scooped).
Mix with your hands until combined, then knead for about 2 minutes (I kneaded it right in the bowl to reduce mess).
Transfer the dough to a 9x13-inch baking dish (or similar size) lined with parchment paper (to prevent the dough from sticking to the pan).
Shape the dough into an oval or rectangle shape, then flatten to approximately 1/2 an inch thickness.
Cover the dish with a lid or aluminum foil, then bake for 45 minutes.
Remove the cover and let the seitan cool to room temperature before removing from the pan and cutting widthwise into 10 slices.
Add 1 1/4 cups chopped dark chocolate to a pot on the stove set to low heat.
Cook, stirring frequently until melted, then immediately remove the pot from the heat.
Dip the seitan slices one at a time in the melted chocolate, gently turning to coat each side, then place them on a sheet pan (or plates) lined with parchment paper (use two forks to make this process easy).
Refrigerate for 1 hour or until the chocolate has hardened.