1. Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.
2. In a large mixing bowl, cream together the unsalted butter and granulated sugar until light and fluffy.
3. Add the eggs one at a time, beating well after each addition.
4. Stir in the vanilla extract and sour cream until well combined.
5. In a separate bowl, whisk together the baking soda, baking powder, salt, and flour.
6. Gradually add the dry ingredients to the wet ingredients, mixing until a smooth dough forms.
7. Drop rounded tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart.
8. Bake in the preheated oven for 10-12 minutes, or until the edges start to turn golden brown.
9. Remove the cookies from the oven and allow them to cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.

For the Frosting:

1. In a small saucepan, melt the butter over low heat.
2. Remove from heat and stir in the powdered sugar, vanilla extract, and milk until smooth.
3. Once the cookies are completely cool, spread the frosting onto each cookie using a butter knife or offset spatula.
4. Allow the frosting to set before serving.