Preheat the Oven: Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat.
Prepare Cookie Dough: In a large bowl, cream together the unsalted butter, granulated sugar, and brown sugar until smooth and fluffy (about 2-3 minutes). Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Dry Ingredients: In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in M&M'S® MINIS: Gently fold in the M&M'S® MINIS into the cookie dough, making sure they are evenly distributed.
Prepare Croissants: Take your baked croissants half the croissants lengthwise.
Assemble the Crookies: On your prepared baking sheet, place a small scoop of cookie dough and press into the center of the croissant.
Bake: Bake the crookies in the preheated oven for 12-15 minutes, or until the edges are golden brown but the center is still soft.
Cool and Serve: Allow the crookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely. This will help the cookies set and maintain their structure.