Preheat oven to 400 degrees. Coat a 9-by-13-inch baking dish with cooking spray, and line with parchment, leaving a 2-inch overhang on long sides. Spray parchment.
Pulse wafers in a food processor until finely ground, and transfer to a bowl. Add sugar and a pinch of table salt. Melt 6 tablespoons butter, and stir into crumbs. Press into bottom of dish. Bake for 10 minutes.
Heat chocolate, condensed milk, whole milk, remaining 4 tablespoons butter, and a pinch of table salt in a double boiler or a heatproof bowl set over a pan of simmering water, stirring, until melted. Stir in vanilla.
Pour chocolate mixture over baked cookie layer, smoothing top with an offset spatula. Sprinkle walnuts and coarse salt over top, pressing slightly to adhere. Refrigerate until firm, about 2 hours.
Remove fudge from dish using parchment to lift, and transfer to a cutting board; remove parchment. Trim edges of fudge with a sharp knife, and cut crosswise into six 2-inch-wide strips. Cut each strip crosswise into 3 equal bars, then cut each bar in half on the diagonal to form 36 triangles.
moved make-ahead note form cooks' notes to special collections
I've made this recipe more times than I can count. It is fast, (although it does need to sit in the fridge for 2 hours to firm up) a cinch. Everyone loves it! I use pink himalayan salt and almonds instead of walnuts -- it looks so pretty! Right now I'm whipping up a batch and I'm going to leave off the salt and try cranberries with toasted almonds on top instead.
These fudge-walnut bars are wonderful! They are very rich and creamy -- and the chocolate crust is a nice addition to the fudge. For best results, use good chocolate and make them a day in advance - two hours of chilling wasn't enough in my opinion - they were better the next day - let them sit overnight in the refrigerator and they will cut beautifully and the texture will be perfect. My stepfather loves fudge -- he'll be getting these for Father's Day!