In a medium bowl, combine butter, powdered sugar, milk (or cream), and vanilla. Use a handheld electric mixer to beat until smooth, adding a little extra milk if necessary.
Candy
Spread a thick layer of buttercream around the top half of your jar or glass. Add a dollop of frosting to the backs of the peeps and place them over the buttercream. Repeat with the M&M's, filling in any open space.
Use a serrated knife to gently saw off the head of the chocolate bunny and set aside.
Milkshake
In a blender, combine ice cream and 1/4 cup of the milk. Blend until smooth. Add up to 1/4 cup more of milk, until milkshake reaches your desired consistency. Pour into prepared jar.
Top with whipped cream, chopped Reese's Egg, and crushed Robin Eggs. Place a straw in the shake and mount the bunny head over it. Serve and enjoy!