Preheat oven to 375°. In a large bowl, cream butter and brown sugar until light and fluffy. Add eggs, 1 at a time, beating well after each.
Combine flour, baking powder, baking soda and salt. Add to creamed mixture alternately with buttermilk and amaretto, beating well after each addition. Stir in almonds.
Fill paper-lined muffin cups two-thirds full. Bake until a toothpick comes out clean, 14-16 minutes. Cool for 5 minutes before removing from pans to wire racks to cool completely.
In a small bowl, beat confectioners' sugar and butter. Add 3 tablespoons cream and 2 tablespoons Amaretto; beat until smooth. Add remaining cream and amaretto if needed to achieve spreading consistency. Frost cupcakes.