Heat oven to 375ºF. In a mixer, beat butter and brown sugar until creamy. Add sour cream, egg and vanilla and beat until lightened, about 1 minute.
Stir together dry ingredients (flour, salt, baking powder, baking soda, oats and cashews). Add the dry ingredients to the sugar and butter mixture and mix until combined. Scrape bottom and sides of bowl to incorporate everything evenly.
Drop 1 inch domes of dough onto parchment-lined baking sheets and bake until golden around the edges and just barely set in the center, 8 to 10 minutes. Let cool.
Make brown butter frosting: Sift the confectioners sugar into a mixing bowl.
In a small saucepan, slowly melt butter over medium heat. Cook, swirling pan occasionally, until golden brown, about 3 minutes. Immediately mix butter, vanilla and milk into the confectioners' sugar. Add more milk depending on the consistency you desire.
Frost cooled cookies. Top with extra cashews if desired. Store in an airtight container for up to 3 days.
Notes
Every oven is different. Adjust cooking time as needed to get golden brown cookies, but don't over bake them.