Cut the tempeh in half, to make 2 squares. Cut each square diagonally into a triangle, to make 4 triangles. Turn each triangle on its side and carefully slice it in half, so you have 2 thinner triangles. Continue in this manner so you have 8 thin tempeh triangles, in all.
Fit a steamer basket into a medium-large saucepan with a tight-fitting lid. Add 2 to 3 inches of water. Gently add the tempeh triangles to the steamer basket. Cover, bring to a boil, and steam the tempeh triangles for 8 minutes. Transfer the triangles to a plate and let cool for 15 minutes.
While the tempeh cools, put the tamari, water, maple syrup, olive oil, vinegar, paprika, and garlic powder into a small bowl and briskly whisk to combine.
Line an 8-inch square, rimmed baking pan with foil. Top the foil with unbleached parchment paper. Arrange the triangles in a single layer in the prepared pan. Pour the sauce over the tempeh triangles, cover and refrigerate for 1 to 4 hours.
When you are ready to bake the triangles, preheat the oven to 200C°/400°F/gas mark 6. Cover and bake for 20 to 25 minutes. Uncover and bake for an additional 20 to 25 minutes or until the triangles are golden brown around the edges. Serve warm.