Combine the butter, sugar and salt in the bowl of a stand mixer fitted with a paddle attachment (or in a large bowl if using a handheld electric mixer). Mix on low speed until mostly incorporated. Add the vanilla; increase the speed to medium-high and mix until smooth. Adjust the consistency with milk as desired. Use immediately or refrigerate in an airtight container up to 3 days. Allow to come to room temperature and mix on medium-high until smooth.
Put 1/3 of the frosting into a medium bowl and the remaining 2/3 into a second bowl. Color the smaller amount of frosting with the green food coloring and stir to combine. Color the larger amount of frosting with 5 to 10 drops of red food coloring – until a dark red color is achieved. Transfer the green buttercream to a disposable piping bag fitted with a #63 leaf piping tip. Transfer the red buttercream to a disposable piping bag fitted with a #366 leaf piping tip.
Pipe a dollop of red buttercream on the tops of each cupcake and smooth with a small offset spatula to cover the tops.
Pipe red buttercream on top of each cupcake creating petals in a star shape. Pipe a second star shape pattern on top of the first but offset the points so they are in between the first layer of petals. Pipe the green frosting with the flat side of the piping tip facing towards the cupcake and make leaves in between each of the bottom layer of petals. Pipe a cross shape with the red frosting in the center of the flowers. Sprinkle the centers with silver dragees.