01 -
Preheat oven to 450°F. Grind steel cut oats into fine flour using a spice grinder. In a stand mixer fitted with the paddle attachment, combine oat flour, all-purpose flour, cornstarch, confectioners' sugar, and salt. Mix on low speed until combined. Add cold unsalted butter and beat on low until dough forms and pulls from the bowl, about 12 minutes. Transfer dough to parchment-lined work surface.
02 -
Form dough into a ball and flatten slightly. Place second parchment sheet atop dough and roll to 3/16-inch thickness. Using a 2 1/2-inch round cutter, cut circles. From half the circles, cut 2 1/4-inch center rings. Transfer full circles to baking sheet, top each with a dough ring, pressing lightly to adhere. Dock centers by pricking four times with toothpick. Gather scraps and repeat rolling and cutting until 24 cookies are formed.
03 -
Bake cookies for 10 minutes at 450°F. Reduce oven to 300°F and continue baking 15 minutes until edges are pale golden, rotating pan midway. Transfer to wire rack to cool.
04 -
Combine sweetened condensed milk, butter, white and brown sugar, corn syrup, salt, and water in a saucepan. Stir constantly until mixture boils. Using instant-read thermometer, cook until 245°F. Remove from heat and stir in vanilla extract. Transfer to heatproof vessel.
05 -
Spoon caramel into the well of each shortbread cookie, allowing it to slightly dome above edges. Excess caramel can be poured into a buttered dish, cooled, then cut into candies.
06 -
Reserve 1/4 cup chocolate. Microwave remaining chocolate in 15-second bursts, stirring until melted and reaches 115°F. Stir in reserved chocolate gradually while cooling to 85°F. Reheat in 10-second increments stirring until 90°F. Avoid heating above 94°F. Repeat if overheated.
07 -
Place cookies on wire rack over parchment. Pour melted tempered chocolate over cookies, allowing excess to drip off. Lightly tap tray for even coating. Immediately sprinkle coarse sea salt atop each cookie. Let chocolate set at room temperature for 10 minutes, then chill 10 minutes. Remove carefully with spatula and store at room temperature in covered container for up to 5 days.