Preheat the oven to 350°F (180°C) and line a 12-cup muffin tin with paper liners.
Add the flour, baking powder, and salt to a bowl and whisk to combine.
Place the butter and sugar into a large mixing bowl. Mix on high speed with an electric mixer for about three minutes, or until light and fluffy.
Add the eggs one at a time, beating between each addition, until very smooth and light in texture.
Add the vanilla and green food colouring and mix to incorporate.
Add half of the milk and mix on low speed until combined. Add half the flour mixture and mix. Repeat, being careful not to over-mix.
Scoop the batter out into the prepared muffin tin, filling each cup three-quarters full. Bake for 18-20 minutes, or until lightly golden in colour and the cupcakes pass a toothpick test.
Cool the muffins for about 10 minutes in the tin before removing to cool fully on a rack.
The cupcakes will keep in an airtight container at room temperature for 3-4 days, and freeze well. After frosting, it’s best to refrigerate (take them out half an hour before serving to soften the icing).
To Decorate
Make the buttercream, then divide into three parts to add food colouring. You need about ¼ cup each of yellow and black, and the remaining should be coloured green.
Add the food colouring until the icing reaches the desired shade, then scoop into piping bags.
For the green top, use an offset spatula or butter knife to spread an even layer of icing, creating a flat top.
Use a swirl piping tip to pipe a swirl of green 'hair' at the top of the cupcake.
Pipe the yellow eyes with a small petal tip, the larger part closer to the middle and moving outward to create a teardrop shape.
Use a very small round tip for the black icing. Pipe two small dabs on the yellow eyes, followed by eyebrows, a half circle for the nose, and the smile.
Optionally add a candy heart, and serve.
Video
Notes
See the attached video for how to decorate as a 3D Grinch with a hat.