For the cookies: Preheat oven to 350°F (170°C). Line baking tray with parchment paper, if desired. Beat brown sugar and butter until creamy. Add egg and vanilla and beat to combine. In a separate bowl, stir together flour, baking powder, baking soda and salt. Add flour mixture alternately with sour cream to butter-sugar mixture, beating on low speed until combined. Fold in cashews.

Drop tablespoonfuls of dough 2 inches (5 cm) apart onto baking sheet and bake for about 10 minutes until cookies are golden brown. Cool cookies on a wire rack.

For the frosting: For the frosting, heat butter in a large saucepan over medium heat until boiling. Continue to heat the butter, swirling occasionally, until the sediments (solids at the bottom of the pan) turn golden brown. Remove from heat and whisk in powdered sugar, vanilla extract or vanilla sugar and milk. Immediately spread/spoon the brown butter frosting over the cookies. Top with a cashew half. The frosting can get a bit weird and separate but don’t worry. You can whisk in a little more powdered sugar and milk and that will bring it back.

Stored tightly covered cookies will stay fresh for several days.