Preheat oven to 350°F (177°C). Line baking sheets with parchment paper.
Whisk flour, cornstarch, baking soda, and salt.
Cream butter with sugars until light and fluffy.
Add eggs one at a time, then vanilla; mix well.
Gradually add dry ingredients, mixing until just combined.
Fold in M&M’s, chocolate chips, and sprinkles.
Scoop 2 tablespoons dough, roll into balls, shape taller to minimize spreading.
Bake 11-12 minutes until edges are lightly golden and tops dry.
Immediately press extra decoration toppings on warm cookies; cool 10 minutes, then transfer to wire rack.
For filling, beat butter until creamy; add powdered sugar, salt, vanilla, and milk; beat until fluffy.
Divide frosting into three parts; tint two with green and red gel colors, leave one plain.
Pipe a dollop of each color onto half the cookies; top with remaining cookies and gently press to sandwich.
Serve immediately or store covered.