2 cups old-fashioned rolled oats (not quick-cooking)
1 cup all-purpose flour, spooned and leveled
¾ teaspoon baking soda
¼ teaspoon ground cinnamon
2 teaspoons sea salt
½ cup (1 stick) unsalted butter, at room temperature
4 tablespoons vegetable shortening
½ cup granulated sugar
½ cup packed light brown sugar
1 large egg
1 teaspoon pure vanilla extract
6 ounces bittersweet chocolate, coarsely chopped
Heat oven to 375° F with the rack in the upper and lower thirds. Line 2 large baking sheets with parchment.
Combine the oats, flour, baking soda, cinnamon, and 1 teaspoon of the salt in a medium bowl; set aside.
Beat the butter, shortening, granulated sugar, and brown sugar with an electric mixer on medium-high speed until lightly and fluffy, 2 to 3 minutes. Beat in the egg and vanilla. Reduce the mixer speed to low and gradually add the flour mixture, mixing just until combined (do not overmix). Stir in the chocolate.
Drop heaping tablespoonfuls of the dough onto the prepared baking sheets, spacing them 2 inches apart. Sprinkle with the remaining 1 teaspoon of salt.
Bake, rotating the pans halfway through, until lightly brown around the edges, about 10 to 12 minutes. Cool slightly on the baking sheets, then transfer to a wire rack to cool completely.