Preheat oven to 350°F. Grease an 8×8" baking dish and set aside.
In a large mixing bowl, combine sugar, pumpkin, coconut milk, and vanilla. Use a sieve to sift in the flour, baking powder, salt, and spice (if using) then mix until well combined. Add chocolate chips and briefly mix to combine.
Add batter to baking dish. If your batter has lumps in it, pour some batter in the sieve and use a spatula or spoon to press the batter down, smoothing out any clumps. Repeat with remaining batter.
Tap the pan on the countertop a few times to release air bubbles from the batter. Bake for 45-60 minutes, until the top begins to crack and the edges are slightly brown.
Let the mochi cake cool completely to room temperature before slicing. Store leftovers in an airtight container at room temperature for up to 3 days.