Line an 8-inch square baking dish with parchment paper and grease with butter or nonstick cooking spray.

In a large mixing bowl, whisk together the flour, cream of tartar, baking soda, and salt; set aside.

In the bowl of a stand mixer fitted with a paddle attachment, beat the butter until smooth, about 1 to 2 minutes. Add the brown sugar and 1/2 cup granulated sugar and continue beating on medium speed until smooth and creamy.

Scrape down the sides of the bowl, add the egg, and beat just until combined. Add the milk and vanilla and beat on medium-high speed. Dump in the flour mixture and beat on low for about 1 minute.

Dollop half of the dough into the baking dish. Wrap the other half in plastic wrap. Place both in the freezer for 15-20 minutes so that dough is less sticky and easier to work with. In a small dish, combine the 1/4 cup of sugar and 2 teaspoons of cinnamon with a fork or spoon.

Preheat oven to 350ºF. Once chilled, press the dollops of dough into an even layer in the bottom of the baking dish. Sprinkle with half of the cinnamon-sugar. Unwrap the chocolate bars, then place side by side on top of the dough. Dollop marshmallow creme on top of chocolate bars and spread evenly.

Take the remaining dough, flatten into discs*, and layer them side by side on top of the marshmallow creme. Try to cover as much of the marshmallow creme as possible. Sprinkle with the last half of the cinnamon-sugar.

Bake for 27-30 minutes, or until edges are lightly golden. Cool completely**. Using the parchment overhang, gently lift the bars out of the pan and cut into squares. For cleaner looking bars, I recommend cutting just before serving since the marshmallow creme tends to run a little. Store bars in an airtight container.