For the base:
2 cups all-purpose flour
1/2 cup light brown sugar, packed
½ tsp kosher salt
1 ½ sticks cold butter, cut into ½-inch cubes
For the topping:
2 sticks butter
1 2/3 cups granulated sugar
¼ cup light corn syrup
½ tsp salt
1½ cups fresh (or frozen (but not thawed)) cranberries (6 ¾ oz), coarsely chopped
1 tsp vanilla
3 cups pecans (12 oz), toasted and cooled, then coarsely chopped
2 oz bittersweet chocolate (not semi-sweet or unsweetened), very finely chopped
a candy thermometer
To make the base: Preheat the oven to 350°F and arrange a rack in the middle position.
Line a 9 x 13-inch shallow baking pan with foil, leaving a 2-inch overhang on the two short sides. Butter all four sides, but not the bottom.
Add the flour, brown sugar and salt to the bowl of an electric food processor. Pulse a few times to combine. Add the cubed butter and pulse until the mixture begins to form small, pea-sized lumps. Transfer the dough to the prepared baking pan, then press down firmly with your fingers and the heel of your hand to form an even layer. Bake until lightly golden and firm, anywhere between 15 to 25 minutes. Cool on a wire rack to room temperature.
To make the topping: Melt the butter in a medium-sized sauce pan over medium heat. Stir in the sugar, corn syrup and salt. Increase the heat to medium-high and boil, stirring occasionally, until the mixture is deep golden brown and registers 245°F on a candy thermometer, about 8 minutes. (If it takes longer, so be it – The important thing is the temperature, not the time.) Carefully add the cranberries and stir to combine. Continue to boil until the caramel returns to 245°F. Remove the pot from the heat and stir in the vanilla. Then add the pecans and stir until well coated. Working quickly, spread the caramel topping over the cookie base, using a rubber spatula to distribute the sauce and a fork to distribute the nuts and berries evenly. Cool completely to room temperature.
Once cool, lift the bars by the foil from the pan and transfer to a cutting board. Cut in 6 cross-wise strips, then 6 length-wise strips, for a total of 36 bars. (I like to sliced down the center first, and then slice each half in three even pieces. Doing this in both directions gives me the most even results.)
Place the chocolate in a small heat-safe bowl placed over a pot with about an inch of simmering water. (Make sure the bowl fits snugly inside the pot, or the chocolate will burn from the direct heat of the stove-top.) Stir until melted and smooth. Transfer the chocolate to a small heavy-duty plastic bag. Using a scissors, snip off a tiny bit of one corner to form a small hole. Evenly pipe the chocolate in wavy stripes across the bars. Let stand at room temperature until the chocolate sets, about 30 minutes to 1 hour. Store at room temperature in an air-tight container, up to 3 days.