Preheat the oven to 350°F. Line a muffin pan with paper liners. Set aside.
In a small bowl, combine the milk and 1/2 Tablespoon of vinegar. Set aside to “sour”.
In a large bowl, cream together the 4 Tablespoons butter and white sugar until light and fluffy. Add in the egg, and beat until fully combined. Mix in the cocoa powder and vanilla.
Slowly add in half of the “soured” milk. Add in half of the flour, and mix until combined. Add in the remaining milk and flour until fully incorporated.
Add in the baking soda and vinegar, and mix until smooth. Add in the food colouring until your desired shade.
Divide the batter evenly into the prepared muffin pan, and bake in the preheated oven or 20 minutes, or until a toothpick inserted comes out clean.
Allow to cool in the pan for 5-10 minutes, then transfer to cool on wire racks.
To make the frosting: Cream together the butter and cream cheese on high until light and fluffy. Reduce the speed, and add in the powdered sugar and vanilla. Mix until fully incorporated. Place in a ziplock bag or a piping bag, and pipe the frosting onto the cupcakes. Sprinkle tops of cupcakes with the heart sprinkles.