Whisk together the flour, baking powder, and salt. Set aside.

Using an electric mixer on medium speed, beat the butter, sugar, and brown sugar until light and fluffy. Add the eggs and egg yolk, one at a time, mixing well after each addition. Mix in the vanilla.

Reduce the mixer's speed to low. Gradually add the flour mixture, mixing just until combined. Set aside about a cup of the dough to use for the topping.

Spread the cookie dough evenly in the prepared pan. Scatter about half of the M&Ms over the top of the dough, lightly pressing them into the dough. Next, dollop the marshmallow creme on top of the dough. Finally, drop small portions of the reserved dough on top.

Bake for 25 to 30 minutes, or until the bars are golden brown and a pick inserted into the center comes out clean.

Place the pan on a wire rack, and allow the cookie bars to cool for about 5 minutes. Scatter the remaining M&Ms over the bars, gently pressing them into the bars. Top with sprinkles. Let the bars cool completely before lifting from the pan and cutting into bars.