Step 1

Place a rack in middle of oven; preheat to 350°. Toast pecans on a small rimmed baking sheet, tossing halfway through, until slightly darkened and fragrant, 8–10 minutes. Let cool.

Step 2

Line another small rimmed baking sheet with a Silpat baking mat. Cook granulated sugar, butter, and 2 Tbsp. water in a small saucepan over medium-low heat, stirring gently with a heatproof rubber spatula, until sugar is dissolved. Increase heat to medium and bring syrup to a rapid simmer. Cook, without stirring, swirling pan often, until syrup turns a deep amber color, 8–10 minutes. Immediately remove saucepan from heat and stir in pecans. Once pecans are well coated, add baking soda and salt and stir to incorporate (mixture will foam and sputter as baking soda aerates caramel). Working quickly (it will harden fast), scrape mixture onto prepared baking sheet and spread into a thin layer. Let cool completely, 5–10 minutes. Chop into pea-size pieces; set aside.

Step 3

Place half of butter (½ cup) in the bowl of a stand mixer. Bring remaining butter to a boil in a small saucepan over medium-low heat, stirring often with a heatproof rubber spatula. Cook, scraping bottom and sides of pan constantly, until butter sputters, foams, and, eventually, you see browned bits floating on the surface, 5–7 minutes. Pour brown butter over butter in stand mixer bowl, making sure to scrape in all the browned bits. Let sit until butter begins to resolidify, about 30 minutes.

Step 4

Pulse flour, salt, and baking soda in a food processor to combine. Add half of reserved pecan brittle and 1 cup oats; process in long pulses until oats and brittle are finely ground.

Step 5

Add brown sugar and granulated sugar to butter and beat with paddle attachment on medium speed until light and smooth but not fluffy, about 2 minutes. Scrape down sides of bowl and add eggs and vanilla. Beat until very light and satiny, about 1 minute. Scrape down sides of bowl and add flour mixture; beat on low speed until no dry spots remain and you have a soft, evenly mixed dough. Add remaining half of brittle and remaining 1 cup oats; mix on low speed just to distribute. Fold batter several times with a spatula to ensure everything is evenly mixed.

Step 6

Using a 2-oz. scoop or ¼-cup measuring cup, scoop level portions of dough to make 18 cookies. Place on a parchment-lined baking sheet, spacing as close together as possible (you’ll space them out before baking). Cover tightly with plastic wrap and chill at least 12 hours and up to 2 days. (If you’re pressed for time, a couple hours will do; cookies just won’t be as chewy.)

Step 7

When ready to bake, place racks in upper and lower thirds of oven; preheat to 350°. Line 2 large rimmed baking sheets with parchment paper. Arrange 6 cookies on each prepared baking sheet, spacing at least 3" apart.

Step 8

Bake cookies, rotating baking sheets top to bottom and front to back after 12 minutes, until dark golden brown around the edges, 16–20 minutes. Let cookies cool 5 minutes on baking sheets, then transfer cookies to a wire rack with a spatula and let cool completely.

Step 9

Carefully move a rack to middle of oven. Arrange remaining dough on one of the baking sheets (it’s okay if it’s still warm). Bake as before (this batch might go a bit faster).Do ahead: Dough can be formed 2 months ahead; chill dough balls at least 2 hours before transferring to freezer. Once frozen solid, store in resealable plastic freezer bags and keep frozen. No need to thaw before baking, but you may need to add a minute or two to the baking time. Cookies can be baked 5 days ahead; store airtight at room temperature.