Preheat oven to 350 degrees. Grease a 9x13 cake pan with cooking spray. Line the bottom of the pan with a piece of parchment paper and spray grease to cover it. I used two medium sized heart pans.
In a large mixing bowl, add in butter. Mix on medium until creamy.
Add in the granulated sugar and mix on medium until just combined.
Add in the egg and vanilla and continue mixing until the egg is completely incorporated.
Add in the flour and mix on low until a ball of dough forms. The dough shouldn't be too sticky.
Roll out the cookie dough or press it into the baking pan. If you use heart shaped pans, the dough can be divided into two and pressed into each pan. Press the m&m's into the cookie dough.
Spread the sprinkles on the cookie dough. Place into the oven for 15 minutes if you're using two small baking pans. If you're using a 9x13 pan, bake for 20-25 minutes until the center is fully cooked and the sides are slightly brown.
Let cool in the pan for 15 minutes and turn over onto a cooling rack. You can break apart the cookie while it's still slightly warm, or wait until it's completely cool. I first drizzled the melted white chocolate over my bark before breaking it. I also added sprinkles on half a batch after I drizzled the bark with the white chocolate. You could also dip the pieces of bark in some white chocolate or milk chocolate. The options are endless.