Preheat oven to 350ºF.
Cut butter into 1⁄2-inch cubes. Separate the egg white from the yolk and discard the yolk. Place the egg white into a small bowl and set aside.
Whisk the butter in a large bowl until creamy. Add egg white and continue to mix until fully incorporated.
Add the sugar cookie mix and blend with a spoon or mixer until dough is formed.
If you used a standing mixer, place the cookie dough in a medium bowl and cover with plastic wrap.
If you used a bowl and mixed by hand, cover with plastic wrap and place in the refrigerator. Refrigerate for 20 minutes.
Combine granulated sugar, brown sugar, and cinnamon in another small bowl. Set aside.
Remove cookie dough from refrigerator and using your hands, roll into tablespoon-sized rounds and toss them in the cinnamon-sugar mixture to coat. Set aside for the next step.
Place the snickerdoodle cookies onto a parchment-lined sheet pan and press lightly with fingers to flatten. You may have extra of some ingredients. Place in oven and bake for 10 to 12 minutes or until edges are golden.
Remove from oven and cool the sheet pan on a wire rack. Place the Snickerdoodles on a serving platter and enjoy!