Preheat the oven to 325 degrees F.
Spread the pecans on a baking sheet. Bake until richly browned and toasty smelling, about 10 minutes. Let cool, then scoop into a food processor and coarsely chop by turning the machine on and off. Remove about 1½ cups of the nuts and put in a large bowl to use in the filling. Add half of the Mexican chocolate to the nuts in the food processor and pulse the machine to mix them. Add the bread slices; process until everything is fairly fine crumbs. Add ⅓ cup of the melted butter and ¼ teaspoon of the salt. Process just enough to moisten everything (if you don’t have a food processor, you can chop each item separately with any other appliance or gadget you deem appropriate, then combine them in a bowl with the melted butter and salt). Liberally butter a 13-by-9-inch baking pan, then evenly pat in the crumb crust mixture onto the bottom and sides. Refrigerate while you make the filling.
Add the remaining half of the Mexican chocolate, the chopped semisweet chocolate, and the flour to the bowl with the reserved pecans. In the food processor (you don’t need to clean it in between), mix the eggs and sugar until well combined. Add the corn syrup, pulse a couple of times, then add the remaining ⅔ cup of melted butter, the remaining ½ teaspoon of salt, and all the vanilla. Process to combine thoroughly, then pour over the pecan filling mixture, stir well, and scrape everything into your crust-lined pan.
Bake until the bars have pulled away slightly from the side of the pan, 40 to 50 minutes. Let cool to room temperature, then cut into 2-inch squares, dust with powdered sugar, and arrange on an attractive serving platter.