Preheat and Prepare – Preheat the oven to 350°F (175°C). Lightly grease a muffin tin.
Make the Cookie Dough – In a large bowl, beat together granulated sugar, brown sugar, shortening, peanut butter, salt, and vanilla extract until smooth. Add eggs and milk, mixing until fully incorporated. Gradually mix in flour and baking soda until a soft dough forms.
Form the Cookie Cups – Using a cookie scoop, place dough portions into each muffin tin cup. Use a tart press or your fingers to create an indent in the center of each dough ball.
Bake – Bake for 10-12 minutes until golden. Allow to cool in the muffin tin before removing. The centers may rise during baking but will settle upon cooling.
Make the Frosting – In a separate bowl, beat softened butter until creamy. Gradually add powdered sugar, then mix in milk and vanilla extract. Add green food coloring and continue mixing until smooth.
Decorate – Pipe the green frosting onto the cooled cookie cups using a grass or star piping tip. Place 2-3 peanut butter M&M's or candy eggs on top of each frosted cookie cup.
Serve and Enjoy – Let the frosting set before serving. Store leftovers in an airtight container.