Heat oven to 350° F. Butter a 9-by-13-inch baking dish and line with parchment, leaving an overhang on the two long sides; butter the parchment. Whisk together the flour, cocoa, baking powder, and salt in a medium bowl; set aside.
Spread the pecans on a rimmed baking sheet and toast, tossing occasionally, until fragrant, 7 to 10 minutes. Cool, then coarsely chop.
Combine the butter, semisweet chocolate, and bittersweet chocolate in a large microwave-safe bowl. Microwave on high in 30-second intervals, stirring between each, until melted and smooth. Cool slightly. Whisk in the sugar, eggs, and vanilla until smooth.
Add the flour mixture and mix until just combined (do not overmix). Fold in the pecans.
Spread the batter in the prepared pan and drop spoonfuls of the dulce de leche on top. Use a skewer or knife to gently swirl the dulce de leche into the batter. Sprinkle with the pretzels. Bake until the top is set and a toothpick inserted in the center comes out clean, 40 to 50 minutes. Cool completely in the pan.
Holding the paper overhang, lift the brownies out of the pan and transfer to a cutting board. Cut into 24 squares.
Storage suggestion: Keep the brownies at room temperature, between sheets of wax paper or parchment in an airtight container, for about 5 days.