Preheat your oven to 350°F (175°C).
Cream the softened unsalted butter and brown sugar together until light and fluffy.
Add the egg and vanilla extract, mixing until fully combined.
In a separate bowl, whisk together flour, baking powder, and baking soda. Gradually add to the wet ingredients.
Fold in the sour cream and chopped salted cashews until the dough is thick and slightly sticky.
Scoop rounded balls of dough onto a baking sheet lined with parchment paper. Bake for 10-12 minutes.
Cool cookies on the baking sheet for 5 minutes, then transfer to a cooling rack.
To make the frosting, melt the unsalted butter in a saucepan over medium heat until golden brown.
Mix the browned butter with powdered sugar, milk, and vanilla until smooth and creamy.
Frost the cooled cookies with a generous layer of brown butter frosting.
