Preheat your oven to 350°F (175°C) to ensure even baking and perfect gooey cookies.
In a large mixing bowl, combine the softened butter, granulated sugar, and brown sugar. Beat with an electric mixer for 2-3 minutes until light and fluffy.
Add the large egg and pure vanilla extract to the butter-sugar mixture. Mix until well combined and smooth.
In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, chocolate pudding mix, baking soda, and salt to evenly distribute dry ingredients.
Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Avoid overmixing.
Using a spatula, gently fold in the semi-sweet chocolate chips, M&M's, and white chocolate chips until evenly distributed.
Use a cookie scoop to drop rounded tablespoons of dough onto lined baking sheets, leaving space for spreading. Bake for 10-12 minutes until edges are set but centers remain soft.
Remove cookies from the oven and cool on baking sheets for 5 minutes before transferring to a cooling rack to maintain chewy texture.