See this post for instructions on roasting a beet. Otherwise, you could sub another fruit puree, such as butternut squash or applesauce (adjusting added sugar depending on fruit's sweetness).
Butter two standard size muffin tins with dairy-free butter and coat with cocoa powder - shake out excess (as original recipe is written // use fewer or more tins if adjusting batch size). Preheat oven to 375 degrees F (190 C).
Add the almond milk and vinegar to a small mixing bowl and mix. Let set for a few minutes to activate.
Add the sugar, oil, vanilla, and beet puree and beat until foamy. Then add cocoa powder, flour, baking powder, and salt. Mix until no large lumps remain.
Lastly, add the melted semisweet chocolate and mix once more.
Divide the batter evenly between the two muffin tins - it should come up about to the top, which is OK. Break up one square of the dark chocolate and push it down into the centers of the cakes. Cover with batter using a spoon. Bake 15-20 minutes, or until the edges have pulled away slightly and the top no longer appears wet. It's OK if it's a little fudgy but you definitely don't want underdone cake.
Let rest in the pan for 4-5 minutes before removing. Then gently loosen the edges with a butter knife and top with a cutting board or plate and carefully invert. Gently transfer to serving plates. Immediately top each cake with the remaining square of vegan dark chocolate (for extra gooey-ness - optional) and serve with coconut whipped cream. Dig in.
Other garnishes might include a dusting of cocoa powder, powdered sugar, fresh berries, fresh mint, or edible flowers.
*Nutrition information is a rough estimate for 1 cake topped with extra chocolate and coconut whipped cream.