1Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 13x9-inch pan with baking spray with flour.
2In large bowl, beat cake ingredients with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour into pan. Bake as directed on box for 13x9-inch pan. Cool completely, about 1 hour.
3Meanwhile, in 2-quart saucepan, melt butter over medium heat. Stir in brown sugar. Heat to boiling, stirring constantly. Reduce heat to low; boil and stir 2 minutes. Stir in milk; return to boiling. Remove from heat. Cool to lukewarm, about 30 minutes.
4Gradually stir powdered sugar into brown sugar mixture. Place saucepan of frosting in bowl of cold water; beat with spoon until smooth and spreadable. If frosting becomes too stiff, stir in additional milk, 1 teaspoon at a time, or heat over low heat, stirring constantly. Frost cake. Sprinkle evenly with cashews and pecans. Store loosely covered.