Line an 8 by 8 pan with foil.
Over medium heat in a saucepan, melt the white chocolate, sweetened condensed milk, and vanilla together.
Pour into the pan and let cool overnight.
Carefully remove the fudge from the pan and place on a cutting board. Cut with the bunny cookie cutter, being very careful as you remove the fudge from the cutter.
Insert a toothpick into the bottom of the bunnies to use to hold in place on top of the cupcake.