Preheat the oven to 350 F/180 C and line an 8x8 square baking pan with parchment paper so it hangs over all sides.
Cook the sugar and cream of tartar over medium-high heat in a stainless-steel pan until the sugar starts to melt.Occasionally push (don't stir) the sugar around the pan to make sure it is melting evenly. After about 5 minutes, the sugar should be completely melted and amber in color. Remove the pan from the heat and immediately stir in the butter. It will bubble, which is normal! Keep stirring until the butter is completely combined which will take about 1-2 minutes.
Stir in the heavy cream and salt and place it back over a medium-low heat, stirring until completely combined. Pour the caramel into a heat safe dish and set it aside to cool.
In a large bowl, whisk the egg, egg yolk, sugar, and brown sugar until light and fluffy, about 1 minute.
Melt the butter and chocolate together in a microwave safe dish or pot on the stove until smooth and combined, then stream it into the eggs, whisking constantly until combined.
Mix in the vanilla.
Fold in the all-purpose flour, cocoa powder, and salt until just combined.
Evenly spread half of the brownie batter into the baking pan.
Press 1 cup of the pretzels in an even layer on top of the batter.
Spoon dollops of the caramel on top of the pretzels, saving a few tablespoons for the top of the brownie.
Spread the rest of the brownie batter on top.
Dollop the top with the rest of the caramel and use a toothpick to swirl it as best as possible into the batter.
Press the rest of the pretzels on top, then bake for 30-33 minutes or until a tooth pick inserted into the center comes out covered in clear caramel and a few moist chocolate crumbs. Let cool completely on a wire rack then slice with a serrated knife and enjoy!