Preheat your oven to 350°F and line your baking pans with parchment paper. While the oven heats, bring your egg to room temperature (this helps it incorporate smoothly into the dough for better texture). Measure out all your dry ingredients and set them aside—I always sift mine together to avoid lumps, which helps create more tender cookies.
Melt the butter in a small saucepan over medium-low heat, then remove from heat and let it cool for 5 minutes—this prevents the egg from cooking when you add it. In a large bowl, combine the cooled butter with both sugars, stirring until well blended. Add the room-temperature egg and vanilla extract, stirring until the mixture is smooth and creamy. The sugar will help emulsify the mixture, creating a cohesive base for your dough.
Add the dry ingredient mixture (flour, baking soda, baking powder, salt, and cinnamon) to the wet ingredients from Step 2, stirring until just combined. Be careful not to overmix—you want the dough to come together without developing too much gluten, which would make the cookies tough. Once combined, fold in the white chocolate chips and 3/4 cup of M&Ms, being gentle to avoid crushing them.
Using a cookie scoop or spoon, drop rounded portions of dough onto your prepared pans, spacing them about 2 inches apart. Place the pans in your preheated 350°F oven and bake for 10–12 minutes, until the edges are set but the centers still look slightly underdone—they'll continue cooking as they cool. While the cookies bake, use a sharp knife to slice extra M&Ms in half lengthwise; you'll need these for the bunny ears.
As soon as the cookies come out of the oven, they'll be soft enough to press candies into them. For each cookie, place two M&M halves at the top (flat side down) to create bunny ears, and press a whole M&M in the center below the ears to form the bunny face. Let the cookies cool on the pan for 5 minutes so the candies set, then carefully transfer them to a wire rack to cool completely. I find that pressing the candies in immediately while the cookies are hot helps them stick perfectly without sliding around.