Makes one 8-inch (20 cm) pan.
Cuts into 16 dessert portion bars or 25-36 nibbles.
Cook's Note: Click here for Anna Olson's Butter Tart Ice Cream recipe.
Preheat the oven to 350ºF (180ºC). Lightly grease and line an 8-inch (20 cm) square pan with parchment paper so that the paper comes up the sides.
Stir the flour and icing sugar together in a bowl and use a box grater to grate in the cold butter, followed by a fork or pastry cutter to work the mixture until crumbly and pieces of butter are no longer visible (the mixture won’t come together like a dough.) Spread this into the prepared pan and press down and up the sides just a little bit. Bake for about 20 minutes, until it just begins to brown at the edges. Cool completely before filling.
Sprinkle the walnut pieces and chocolate chips over the cooled crust in an even layer. In a large mixing bowl, whisk the brown sugar, corn syrup and melted butter. Whisk in the eggs, white vinegar and vanilla until smooth. Pour this carefully over the nuts and chocolate chips and bake for 30-40 minutes until set, and beginning to bubble at the edges. Cool the pan on a rack and, while still a little warm, lift the parchment (with the square) out of the pan so that it can be easily removed once chilled (the sugar sets and sticks once cooled). Chill this in the pan before slicing and once sliced, the squares can be stored at room temperature.
To get precise squares, trim away the edges (save for butter tart ice cream, see Cook’s Note) and cut when chilled.