Preheat the oven to 350°F (175°C) and line the muffin tin with paper cupcake liners.
In a medium mixing bowl whisk together the flour, baking powder, and salt until well combined.
In a large mixing bowl beat the softened butter and granulated sugar with a mixer on medium speed until light and fluffy, about 2 to 3 minutes.
Add the eggs to the butter mixture one at a time, beating well after each addition, then mix in the vanilla extract.
In a separate small bowl stir together the milk and sour cream until smooth.
Add the dry ingredients to the butter mixture in two additions, alternating with the milk mixture, beginning and ending with the dry ingredients, mixing on low until just combined.
Divide the batter evenly among the 12 cupcake liners, filling each about two-thirds full.
Bake the cupcakes for 16 to 18 minutes, or until a toothpick inserted in the center comes out clean.
Transfer the muffin tin to a wire cooling rack and let the cupcakes cool in the pan for 10 minutes.
Remove the cupcakes from the pan and allow them to cool completely on the wire rack for at least 35 minutes.
To make the frosting, beat the softened butter in a large mixing bowl until creamy and pale, about 2 minutes.
Gradually add the powdered sugar, 1/2 cup at a time, beating on low at first and then increasing to medium until fully incorporated.
Add the heavy cream, vanilla extract, and pinch of salt, then beat on medium-high speed for 2 to 3 minutes until light and fluffy.
Divide the frosting evenly into three medium bowls.
Tint one bowl of frosting with green gel food coloring, one with pink, and one with yellow, adjusting the color intensity as desired and mixing until uniform.
Fit three piping bags with the grass tip, leaf tip, and small round tip respectively, then fill each bag with a different color of frosting.
Using a small knife, spread a thin layer of any leftover frosting on top of each cooled cupcake to help decorations stick.
Sprinkle the centers of some cupcakes with chocolate cookie crumbs to resemble soil.
Pipe green grass over the tops of some cupcakes using the grass tip, covering the surface fully.
Pipe small flowers and buds using the pink and yellow frostings with the round and leaf tips to create a garden look.
Decorate with gummy worms and flower-shaped sprinkles as desired to complete the garden theme.