Preheat the oven to 350 degrees.
Spray two or three ramekins (depending on the size you have) with cooking spray, then line the bottom with a round piece of parchment paper.
In a medium size bowl, beat together the butter and sugar with a hand mixer until smooth.
Add the egg and vanilla, beating until well combined.
In a small bowl, whisk the flour, salt, and baking soda.
Add half of the flour mixture to the batter, beating for a few seconds before stirring in half of the milk and vinegar.
Add the remaining dry ingredients, then beat the remaining milk.
Spread the batter evenly into the ramekins and bake for 30 minutes, or until a inserted toothpick comes out clean.
Flip the mini cakes onto a cooling rack and allow them to completely cool before frosting.
To make the chocolate buttercream, cream the butter and powdered sugar until light and fluffy.
Mix in the melted chocolate and cocoa powder a little at a time.
Keeping mixing until the frosting is smooth and creamy.
Once the cakes have completely cooled, assemble the cakes with frosting between each layer.
Press the crispies onto the sides of the cake and decorate as desired!
