Preheat your oven to 350 F, then grease and line an 8x8 baking pan with parchment paper, leaving paper handles coming up two sides.
Cream together the butter and brown sugar in a stand mixer or using a hand mixer. In a separate bowl, sift together the flour, salt, and cream of tartar. Mix together the topping, set aside.
Add the eggs one at a time, scraping the bowl after each addition. Stir in the vanilla.
Mix the dry ingredients into the wet ingredients and stir until just combined. Pour the batter into the prepared baking pan and try to spread as evenly as possible. Pipe the dulce de leche onto the batter in lines and swirl or draw peaks in the lines using a toothpick, chopstick, or butter knife.
Sprinkle the topping evenly over the bars.
Bake the bars for 20-25 minutes until the edges are just starting to turn a light golden brown. Remove and allow to cool in the pan 10-15 minutes before gently lifting out of the pan (this is where the parchment paper handles come in handy) and placing on a cooling rack to cool completely.
Cut into your desired size and enjoy! The bars will keep in an airtight container at room temperature 3 days, in the refrigerator 1 week.