Step #1 Line a 9-inch square baking pan with foil wrap, extending foil wrap over edges of pan.
Step #2 Butter foil wrap; set pan aside.
Step #3 In a bowl, 3 tbsps brown sugar, mix flour, & baking powder; use a pastry mixer & cut in ½ c butter until mixture resembles coarse crumbs.
Step #4 Press mixture into bottom of prepared pan; bake this at 350° for about 25 mins or until golden brown.
Step #5 In a saucepan, melt the remaining ¼ c butter; stir in the sugar, corn syrup, water, brown sugar, & salt.
Step #6 Stir in cashews; bring mixture to boiling over med-high heat, stirring constantly.
Step #7 Boil, for 5 mins, uncovered, stirring often.
Step #8 Remove pan from heat; stir in the whipping cream & vanilla.
Step #9 Spread butterscotch mixture evenly over baked crust.
Step #10 Bake 12-15 mins more or until most of the surface is bubbly.
Step #11 Cool completely in pan on a wire rack.
Step #12 Using foil wrap, lift bars out of pan; carefully peel foil wrap from sides of bars; cut into bars.
Step #13 To store: place this in layers separated by waxed paper in an airtight container; cover; store at about room temp up to 3 days or freeze up to 3 months.