Heat the oven to 350°F. Line a 9×13-inch baking pan with foil, leaving enough excess on the sides to lift the bars out later. Coat the foil generously with nonstick cooking spray.
Place the melted, cooled butter into the bowl of a stand mixer along with the granulated sugar and brown sugar. Beat on medium-high speed until the mixture looks pale and fluffy.
Add the eggs and vanilla extract while the mixer is running. Continue beating until the wet ingredients are thoroughly blended into the butter-sugar mixture.
Reduce the mixer speed to low. Gradually add the flour, baking soda, and salt, mixing only until the dough comes together.
Fold in the Easter M&M’s, mini chocolate chips, and white chocolate chips, holding back a small portion of each for the surface.
Spread the cookie dough evenly into the prepared pan. Press the reserved candies and chocolate pieces over the top.
Bake for 30 to 35 minutes, until the bars are set and lightly golden without becoming too dark. Take care not to overbake so the center stays soft.
Let the bars cool completely in the pan. Once cooled, lift them out using the foil overhang, then slice into squares or rectangles.