Preheat the oven to 350 degrees F and line a cupcake pan with cupcake liners. In a large mixing bowl with an electric hand mixer or in the bowl of a stand mixer with a paddle attachment, cream together the butter and sugar until light and fluffy. Add the large egg to the mixture and beat until creamy.
In a medium bowl, whisk together the flour, cocoa powder, and baking soda.
Alternately, add the sour cream and the dry mixture into the wet ingredients, mixing until smooth.
Add the red food coloring until the desired vibrant red color is achieved.
Fill each cupcake liner about 2/3 full with the red velvet batter.
Bake for 15-17 minutes or until a toothpick inserted into the center comes out clean.
While the cupcakes are cooling, add the vanilla pudding to a small bowl and add green food coloring gel until desired color is achieved.
Core the center of each cupcake with a small knife or apple corere and fill with a generous spoonful of the green vanilla pudding.
For the buttercream, in a medium bowl beat the softened butter and powdered sugar together until creamy. Add green gel food coloring to achieve a Grinch-like green color.
Decorate the filled cupcakes with the green buttercream and top with heart sprinkles.