Preheat oven to 350 degrees Fahrenheit. Line your muffin tins with paper liners and set aside.
To make the cupcakes, whisk the flour, baking powder, baking soda, and salt in a large bowl. Set aside.
In a separate large bowl, beat the eggs and white granulated sugar with an electric mixer until light and fluffy. Add in 2 sticks of softened butter, 3 teaspoons vanilla extract, and 1 teaspoon peppermint extract and mix well. Next add in the milk and mix until combined.
Pour the dry ingredients into the wet ingredients and stir by hand until just combined. Fill the muffin tins ½ to ¾ full with the batter.
Bake for 18-20 minutes in the preheated oven, or until a toothpick inserted in the middle comes out clean. Cool the Grinch Cupcakes completely on a wire rack before frosting and decorating.
To make the peppermint frosting, beat the 4 sticks of softened butter over medium speed with an electric mixer until soft and creamy. Continue to beat over medium speed as you gradually add the powdered sugar. Stop the mixer and scrape down the sides of the bowl with a spatula as needed to fully incorporate all of the sugar.
Once the butter and sugar have come together, add the peppermint extract and vanilla extract. Beat in the heavy cream or milk, a little at a time, until desired consistency is reached. Next add a few drops of the neon green food coloring and mix to combine. Add a little bit at a time until the right amount of Grinch green is achieved.
Add the frosting to a large ziplock bag or piping bag fitted with a round piping tip. If using a ziplock bag, cut off a small corner at the bottom. Pipe the cupcakes in an upwards swirl pattern. Dust with the green sugar sprinkles and heart sprinkles.
To make your Whoville Tree toppers, lay out a piece of parchment paper on a large baking sheet and place 24 pretzel sticks on it, spaced apart. In a microwave safe bowl, melt the green candy melts in 30 second intervals until melted and smooth, stirring after each interval.
Fill a small ziploc bag with the melted green candy melts and snip off a very small corner. Pipe a tree across the pretzels by going in a back and forth motion and curving at the top. Place a heart sprinkle on the tip of each one and allow these to harden. You can also place them in the refrigerator to speed up the setting time.
When completely hardened and ready to serve, stick your Whoville trees in the top of each cupcake. Enjoy!