Preheat oven to 350 degrees. Spread walnuts on a baking sheet; toast until golden and fragrant, 10 to 15 minutes. Transfer to a wire rack to cool.
Place 1 1/2 sticks butter and the cream cheese in the bowl of an electric mixer fitted with the paddle attachment. Mix on low speed until the cream cheese is broken down but butter is still chunky. On low speed, add flour and salt, and mix until crumbly and just beginning to hold together, about 20 seconds. There should still be some small pieces of butter visible. Divide dough into two equal parts. Form each part into a flattened rectangle, and wrap in plastic wrap. Transfer to the refrigerator to chill 5
hours or overnight.
Place the chocolate in the bowl of a food processor. Pulse until the chocolate is very finely chopped, about 7 seconds. Transfer to a large bowl. Finely chop toasted walnuts by hand, and add to bowl. Add sugar, cinnamon, currants, zest, corn syrup, and melted butter, and combine by hand. Beat egg yolk with the water, and set aside.
Have ready a 9-by-13-inch baking pan lined with parchment paper. Place a rectangle of dough between two 9-by-13-inch pieces of waxed paper; roll dough into a rectangle the size of the baking pan. Line prepared baking pan with dough. Spread dough evenly with walnut mixture. Roll remaining rectangle of dough into a rectangle the size of the baking pan; place on top. Trim the edges of the dough so they are even. Brush the top of dough with egg yolk mixture, and sprinkle with sugar. Bake until golden, about 35 minutes, rotating halfway through. Cool on a wire rack. Cut into 50 to 60 rectangles, each 2 1/2 by 3/4 inches.