Prepare the marinade by adding 1/4 cup pure maple syrup, 1/4 cup creamy natural peanut butter, 1 teaspoon pure vanilla extract, and 1/4 teaspoon salt to a 9-inch dish (or similar size), then mix well.
Cut 9 oz tempeh into approximately 1-inch pieces (about 24 pieces total).
Bring a pot of water to a simmer on the stove, then add the tempeh.
Cook for 15 minutes, adjusting the heat as needed to maintain a gentle simmer.
Drain and discard the water, then transfer the tempeh to the dish with the marinade.
Once the tempeh is cool enough to handle, use your hands to gently coat all sides of the tempeh pieces in the marinade.
Refrigerate, uncovered, for at least 8 hours (up to 24 hours is fine).
When the tempeh has finished marinating, melt 1 1/4 cups chopped dark chocolate using your preferred method:Stovetop: add the chopped chocolate to a small pot over low heat. Stir constantly until just melted, then immediately remove from the heat.Microwave: add the chopped chocolate to a microwave-safe bowl. Heat in 20-second bursts, stirring between each, until melted. Add a piece of tempeh to the melted chocolate, gently stir to coat, then use a fork to lift it up, allowing any excess chocolate to drip back, and transfer it to a sheet pan (or plates) lined with parchment paper. Repeat with the rest of the tempeh pieces.
Refrigerate for at least 1 hour or until the chocolate has hardened before serving.