Preheat oven to 325 degrees F (165 degrees C).
Line an 18-cup mini muffin tin with paper liners.
Melt chocolate in a microwave-safe glass or ceramic bowl in 15-second intervals, stirring after each melting, 1 to 3 minutes.
Combine melted chocolate, sweetener, water, butter, and egg in a bowl.
Stir until sugar dissolves.
Add soy flour, mayonnaise, vanilla extract, salt, and baking soda.
Whisk until no chunks remain.
Pour batter into the muffin cups.
Bake in the preheated oven until a toothpick inserted into the center of a brownie cake comes out clean, 18 to 21 minutes.