Line a 13x9 cake pan with foil and then butter the foil.
In a mixing bowl, cream the 3/4 cup butter with the 1/3 cup brown sugar. Whisk the flour and baking powder together. Slowly beat the flour into the butter and sugar. Press evenly into the baking pan.
Bake in a 350F oven for 25 minutes.
Meanwhile, make the butterscotch by melting the 1/3 cup butter in a saucepan on medium heat. Add the white sugar, brown sugar, corn syrup, water, salt. Once the sugars are dissolved, stir in the cashews.
Bring the sauce to a boil on medium/high heat. Boil, stirring often for 5 minutes. Remove from the heat and stir in the whipping cream and vanilla.
Pour the butterscotch mixture over the baked crust.
Place back in the oven and cook another 13-16 minutes. The surface should be bubbling all over. Remove from the oven and cool on a wire rack.