Preheat oven to 375⁰F and lightly grease an 8-inch square glass baking dish with oil.
Whisk flax and water together in a small bowl and set aside for 5 minutes to gel up.
In a large bowl, stir together the oat flour, 3/4 cup sugar, 1/3 cup cocoa powder, cinnamon, ginger, nutmeg, chocolate, salt, and baking powder.
In a small bowl, whisk together the flax mixture, milk, pumpkin, and vanilla.
Pour wet mixture onto dry ingredients and stir until thoroughly combined.
Scoop the batter into the prepared glass dish and smooth out evenly with a spoon.
In a small bowl or mug, combine the remaining tablespoon of cocoa powder and 1/4 cup sugar. Sprinkle all of it evenly over the cake batter.
Slowly pour the hot coffee over the cocoa powder and sugar mixture ensuring that the coffee completely covers the powder and sugar. The cake will now look like a complete disaster, but this is normal. Promise!
Very carefully place the dish into the oven, uncovered. Bake at 375⁰F for 24-30 minutes (I baked for 25 mins., but your oven might need more or less – see my post for the visual step by step pictures) until the cake is semi-firm on the top, but bubbly and gooey around the edges (see photo in blog post). It will look under-baked, but this is normal (we want it saucy!). If for whatever reason your cake is still watery after 30 minutes in the oven, keep baking it until it looks like the photo in the blog post.
Let the cake cool for 10 minutes and then serve immediately with vegan vanilla ice cream and toasted pecans. This cake is best served warm straight out of the oven. I tried reheating leftovers in the oven the next day and the cake absorbed a lot of the sauce during the reheat. It was still good, but we preferred the cake the first day, fresh out of the oven. Leftovers are quite good served chilled, as an alternate suggestion.