Prepare cupcakes - In a mixing bowl, combine cake mix, eggs, water, and oil.. Beat with a hand mixer or the standing mixer for 2-3 minutes until fluffy and well combined.
Line a muffin tin with cupcake liners and fill each liner ⅔ of the way full. Bake according to the cake mix package instructions (approximately 15 minutes at 350°F.)
Allow cupcakes to cool completely.
Use grass piping tip #233 attached to the green tube of icing or a piping bag fitted with the tip to pipe a mound of grass on the top of each cupcake. Hold the tip close to the cupcake and squeeze icing while pulling quickly outward and releasing pressure to make the grass strands.
Cut about ⅓ off of the end of one standard marshmallow to make the bunny butt. (You can also just leave it whole.)
On the intact end of the marshmallow, place a dollop of white icing and dip into the sanding sugar to make a sparkly bunny tail. Press the cut end of the marshmallow into the grass on the cupcake.
Take a second marshmallow and snip on the diagonal to cut off two triangular shaped marshmallow pieces - these will be the two feet - the cut side of each piece will be the bottoms of the feet.
Place the feet into the grass up against the bunny butt.
Use a small round piping tip attached to the pink tube of icing to pipe the pads of the feet. If the dots of icing come to little points, just wait for them to set up a bit (about 5-10 minutes) and then gently press the pad of your finger against the point to flatten it.