Preheat the Oven and Prepare the Pan: Preheat your oven to 350°F. Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal, or lightly grease it.
Cream Butter and Sugars: In a large mixing bowl, cream together the 2 sticks of butter, 1 cup of granulated sugar, and 1 cup of brown sugar until light and fluffy. This process should take about 2-3 minutes with an electric mixer on medium speed.
Add Eggs and Vanilla: Beat in the 3 large eggs, one at a time, ensuring each is fully incorporated before adding the next. Stir in the 1 1⁄2 teaspoons of vanilla extract.
Combine Dry Ingredients: In another bowl, whisk together the 3 cups of all-purpose flour, 3/4 teaspoon of baking soda, and 3/4 teaspoon of salt.
Mix Dry and Wet Ingredients: Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Be careful not to overmix.
Add Chocolate and M&M's: Fold in 1 cup of semi-sweet chocolate chips and 1 cup of Easter M&M's gently into the batter. Reserve some M&M's for topping.
Spread the Batter: Transfer the batter to the prepared baking pan. Use a spatula to spread it evenly across the pan.
Bake: Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean. Be careful not to overbake to ensure the bars remain soft and chewy.
Cool and Slice: Let the bars cool in the pan on a wire rack for at least 10 minutes. Use the parchment paper overhang to lift the bars out of the pan and transfer them to a cutting board.
Make the Frosting: While the bars are cooling, mix the vanilla icing and blue food coloring together in a bowl using a mixer on high. Add the food coloring very slowly so you end up with a light robin’s egg blue. Frost the whole top of the cookie bars once they have cooled.
Make the Egg Shell Look: Mix the water and unsweetened cocoa powder together in a small bowl until smooth then take a silicone pastry brush and VERY lightly splatter the cocoa liquid over top of the blue frosting to give it a robin’s egg look. Do this carefully!
Cut & Serve: Let the frosting and liquid set then cut into squares or rectangles and serve. Sprinkle the reserved Easter M&M's over the top for a festive look.