1. Preheat the oven to 350°F. Line a 9-by-13-inch baking pan with parchment paper, letting an inch of excess paper hang off the sides. Lightly grease the parchment with nonstick spray.

2. In a large heat-safe bowl, combine the chocolate chips with the chopped chocolate, butter, brown sugar and sugar. Place the bowl over a medium pot of simmering water and heat the mixture, stirring occasionally, until the chocolate is fully melted. Set aside to cool slightly, 5 to 10 minutes.

3. Whisk the eggs, egg yolk and vanilla extract into the chocolate mixture. Add the flour and baking powder, and stir to combine. Gently fold in the crushed pretzels.

4. Pour the brownie batter into the prepared pan and use a spatula to smooth it into an even layer.

5. Top the batter with the whole pretzels, forming an evenly spaced grid (4 rows by 6 columns). Garnish with the flaky salt.

6. Bake until the brownies look set on the top and a toothpick inserted into the center comes out clean, 20 to 25 minutes. Cool for at least 30 minutes. Cut the brownies into squares so that each bar has a whole pretzel in the center, and then serve.