Preheat the oven to 400 degrees Fahrenheit. Line a baking sheet with silicone mat or parchment paper, and set aside. Pour Valentine sprinkles into a dish with low sides, and set aside.
Add sliced, cold butter into mixer and beat until smooth (a stand mixer with paddle attachment is preferred but not necessary).
Add brown and white sugar, beat until smooth; just a minute or two.
Add the eggs and beat until just mixed in.
Scoop the flour into measuring cup to measure, level off the top, and dump it into the mixer bowl.
Add the cornstarch, baking powder, baking soda, and salt - mix on low speed just until all is mixed in
Add M&Ms and milk chocolate chips; pulse in just enough to mix throughout the cookie dough.
Using an large spoon or 1/4 cup cookie scoop, scoop cookie dough, but don't pack it down, or roll it into a ball.
Roll the cookie dough in the Valentine sprinkles to cover all the sides.
Set on a cookie sheet, place 3 to 4 more M&Ms on top.
Continue scooping cookies and rolling in Valentine sprinkles, placing 6 cookies per cookie sheet.
Bake in the preheated about 8 minutes, until the sides are lightly golden, and the tops are just set.
Remove the pan from the oven and cool on the baking sheet for 10 minutes. They will keep cooking on the pan. Don't remove them early or the cookies will break.
Remove to cooling rack for additional cooling as needed.
Store extra cookies on counter in an air tight container for up to 1 week.