A dessert for the ages, this crust is made with a sweet-salty-buttery pretzel. Add some caramel flavor by enjoying a Modelo Negra.
Heat oven to 325 degrees. Butter the bottom and sides of two 8 x 8-inch baking pans. Cut a piece of parchment to fit the bottom of each pan, then press them firmly in place. Butter the parchment paper.
Use a food processor to chop the pretzels into fairly fine crumbs. (You should have 2 cups of crumbs.) Melt 2 sticks of the butter in a small saucepan over medium heat or in a microwave at 50% power. Add to the food processor, along with the 1/2 cup of granulated sugar. Pulse until everything is combined. Divide the crumb mixture between the two prepared pans and pat into an even layer covering the bottoms completely.
Put pecans onto a rimmed baking sheet. Bake until noticeably darker and toasty smelling, about 10 minutes. Let cool to lukewarm, then coarsely chop into 1/4 to 1/2-inch pieces. Scrape into a large bowl. Add the 2 chocolates and the flour to the pecans. Stir to combine, then divide evenly between 2 pans.
In your small saucepan or microwave, melt the remaining 2 sticks of butter. In the bowl combine, with an electric mixer, the eggs, brown sugar, corn syrup and vanilla. Beat at medium-low speed (if your mixer has a choice, use the flat beater). Slowly add the melted butter, mixing until the batter looks smooth. Divide the batter between the two pans, pouring it slowly and evenly over the surface to ensure even distribution of the chocolate and pecans through the batter.
Slide into the oven and bake for 45 to 55 minutes, until the center has just set. Let cool to room temperature. Cover and refrigerate until firm for easy cutting. Cut into 2-inch squares. Keep them stored in the refrigerator until just before serving. Transfer to a serving platter. Sprinkle with powdered sugar.