6tablespoonsunsalted butter, cold, cut into small pieces
For the Filling:
1 ½cupswalnuts, toasted and chopped
1cupsemisweet chocolate chips
6tablespoonslight corn syrup
1/4cupgranulated sugar
1tablespoonunsalted butter
1tablespoonunsweetened cocoa powder
1teaspooninstant espresso powder
1/4teaspoonsalt
2eggs, room temperature
1tablespoondark rum
Instructions
Preheat the oven to 350 degrees F. Butter the bottom and sides of a 9-inch square baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend two sides of the pan and overhang slightly on both ends. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.
Spread the walnuts onto a baking sheet and place in preheated oven 5-7 minutes until fragrant. Shake the baking sheet halfway through baking so the nuts will toast evenly. Remove nuts from oven and set aside to cool completely. Once cool, chop nuts and set aside.
Place flour, brown sugar and salt in the bowl of a food processor fitted with a metal blade. Process with six 1-second pulses.
Remove cover of food processor and sprinkle the butter evenly over the dry ingredients. Cover and process with 12 1-second pulses.
Transfer dough prepared baking pan and press evenly into the bottom of the pan. Bake for 15 minutes, or until barely golden brown; cool completely on wire rack.
In a medium heatproof bowl, combine chocolate, corn syrup, sugar, butter, cocoa, espresso powder, and salt. Melt in microwave heating with 30-second bursts, stirring after each interval, until smooth. Add eggs one at a time, stirring well after each addition. Blend in rum. Pour mixture over cooled crust.
Sprinkle walnuts over the chocolate and press into batter. Bake 18-20 minutes, or until bars are set. Cool completely on wire rack then transfer to the refrigerator to chill before cutting into squares.
Holding onto the parchment, lift bars out of the pan and onto a cutting board. Cut into bars and serve.