Preheat oven to 350F. Grease a 13-by-9-inch baking pan.

To prepare the crust, combine flour and sugar in the bowl of a food processor. Process about 10 seconds to combine thoroughly. Add butter and process until mixture is crumbly, about 30 seconds. Transfer crumbs to prepared pan and press firmly to make a compact layer. Scatter nuts evenly over crust and press them in firmly.

To prepare the caramel layer, melt butter in a heavy medium-size saucepan over low heat. Add brown sugar and stir well until thoroughly combined and smooth. Increase heat to medium-high and cook, stirring constantly, until entire surface of caramel is bubbly. Immediately pour hot caramel over nuts and spread evenly with back of a metal spoon. Bake 18 to 20 minutes or until caramel is bubbly.

Remove pan from oven. Wait 1 minute; sprinkle chocolate morsels evenly over caramel. Let stand 2 to 3 minutes for chocolate to melt. Spread chocolate evenly with spatula or back of a metal spoon. Cool completely, then refrigerate 30 minutes to set. Cut into bars with a sharp knife.