Preheat oven to 350 F. Grease a 9-inch round cake pan. Dust with flour; tap out excess. Mix together flour, cinnamon, baking powder, and salt. Beat together butter and sugar at medium speed until light and fluffy. Add eggs, 1 at a time, beating well after each addition. At low speed, alternately beat flour mixture and milk into butter mixture. Stir in carrots and nuts. Pour batter into prepared pan. Bake cake until top springs back when lightly touched and a toothpick inserted in center comes out clean, 40 minutes. Transfer pan to a wire rack to cool for 10 minutes. Turn cake out onto rack to cool completely. To prepare icing, beat together butter and cream cheese at medium speed until completely smooth. Beat in vanilla. Beat in confectioners’ sugar until well blended. To prepare topping, mix together nuts and brown sugar. Place cake on a serving plate. Spread icing over top and sides. Sprinkle with nut mixture.