Preheat the oven to 350 degrees Fahrenheit.
Crush the pretzels in a food processor to create coarse crumbs. Mix the pretzels with the melted butter and brown sugar, then press into the bottom of a lined 8x8 inch baking dish.
Make the brownie batter: Whisk together the melted butter, sugar, eggs, and vanilla. In another bowl, whisk together the flour, cocoa powder, salt, and baking powder. Slowly add the dry ingredients to the wet ingredients, folding gently until just combined. Pour the brownie batter over the pretzel layer in the baking dish, spreading evenly.
Transfer to the oven and bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Let cool for an hour.
Make the caramel: Heat a medium saucepan over medium high heat. Add the sugar and stir constantly until melted and golden brown, about 5 minutes. The sugar will clump together before melting. Watch and be sure to stir constantly to avoid burning the caramel. Quickly whisk in the butter for about 1 minute, melting and combining completely. Slowly drizzle in the heavy cream while whisking constantly for about 1 minute, until combined completely. Remove from heat and stir in the vanilla and salt. Let cool until very thick but still pourable, about 15-20 minutes.
Pour and spread desired amount of caramel on top of cooled brownies. Let chill for at least an hour, until the caramel is mostly set. Sprinkle with a generous pinch of flakey salt.
To prevent the caramel from sticking, run a knife under hot water. Slice into pieces and enjoy!