Preheat the oven to 350 degrees F. Grease an 8-inch square baking pan with butter and line with parchment paper.
For the topping, combine the flour, brown sugar and salt in a medium bowl. Use a fork to cut in the butter until the mixture resembles pea-sized clumps. Chill in the refrigerator.
In a small bowl, combine the cinnamon and granulated sugar.
For the bars, whisk together the flour, baking soda, cream of tartar and salt in a medium bowl. In a large bowl, stir together the melted butter and both sugars. Beat in the eggs one at a time. Fold in the dry ingredients until just combined. Spread half the batter into the prepared baking pan. Sprinkle with half the cinnamon sugar. Spread the remaining batter into the pan. Scatter the topping in an even layer and sprinkle with the remaining cinnamon sugar.
Bake for 30-35 minutes until a toothpick inserted into the center comes out clean. Cool completely in the pan on a wire rack before removing and cutting into 16 bars. Store in an airtight container.