Position racks in the middle and upper third of the oven. Heat the oven to 350°F. Line two baking sheets with nonstick liners, like Silpat brand, or with parchment.
In a food processor, grind the 2 oz. of pecans finely and measure out 1/2 cup. Heat the butter, sugar, and corn syrup in a medium saucepan over low heat, stirring often, until the butter melts and the sugar dissolves. Increase the heat to medium high and, stirring constantly, bring the mixture just to a boil. Remove the pot from the heat and stir in the flour and salt until incorporated. Stir in the vanilla, the ground pecans, and chopped pecans.
Drop the batter by the teaspoon 3 inches apart on the baking sheets, about six cookies per baking sheet. Bake the cookies until evenly light brown, 11 to 12 minutes total. About 5 minutes into baking, switch the sheets from top to bottom and back to front to promote even baking. The cookies won't begin to spread until about 6 minutes into baking.
Line a cooling rack with paper towels. Remove the cookies from the oven and, as soon as they're firm (which will take just a few minutes), use a wide spatula to transfer them to the rack to cool completely.
Melt the milk chocolate in a bowl in a microwave or over a water bath. Let it cool enough to thicken slightly.
Arrange the cookies in pairs of similar size. Turn half of the cookies bottom up. Leaving a 1/2-inch border around the edge, spread a thin layer of milk chocolate over the cookies that are bottom up. Gently place the remaining cookie, bottom down, onto the milk chocolate. Let the cookies sit until the filling firms, about 30 minutes.
The cookies will stay crisp and flavorful for a few days wrapped in waxed paper or plastic wrap at room temperature. They also freeze well in a tin or sturdy plastic container.