Preheat the oven to 350 degrees F (175 degrees C). Line two 12-cup muffin tins with paper liners.

Combine sugar, flour, cocoa powder, baking powder, baking soda, and salt in the bowl of an electric mixer. Mix briefly until combined. Add pumpkin, milk, canola oil, and vanilla extract; mix on medium speed for 2 minutes. Add water and mix until just blended.

Spoon batter into the prepared muffin cups, filling each almost all the way to the top.

Bake in the preheated oven until a toothpick inserted into the center of a cupcake comes out clean, about 20 minutes. Let cool in trays for about 10 minutes before transferring to wire racks to cool completely.