Butter two standard size muffin tins with dairy-free butter and coat with cocoa powder - shake out excess
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' should be a little fudgy but not underdone.
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.