Heat oven to 325. Combine softened butter and brown sugar in a large mixing bowl. Blend well. Add the egg, vanilla and sour cream.
In a separate bowl, blend the flour, baking powder, baking soda, and salt until thoroughly combined. Add the dry ingredients to the butter mixture. When thoroughly combined, add the coarsely chopped cashews and mix to incorporate them.
Drop by rounded teaspoonfuls on an ungreased baking sheet. (I rolled mine but the original recipe doesn't say you have to.) Bake for 12 to 15 minutes. Cool and frost with brown butter frosting.
Melt 1/2 cup of butter in a saucepan over medium heat until lightly browned. Let it cool a minute then add 2 cups of powdered sugar. Stir in whipping cream and vanilla until smooth and creamy. (If it's not creamy enough to easily spread, add a bit more whipping cream.) Add salt to taste.