Beat 1 cup butter and eggs; add sugar, 1 cup milk, poppy seed mixture, baking powder and flour. Mix well. Pour into 9x13 pan sprayed with non-stick spray and bake at 350 for 35 minutes. Remove from oven to cool.
While cake is cooling, make frosting: Melt 1/2 cup butter in saucepan, add the 1 cup brown sugar and bring to a boil and boil over low heat for 2 minutes, stirring constantly; add milk, and bring back to a boil, stirring constantly. Remove from heat and cool; then add the 2 cups powdered sugar and beat well. Spread on cooled cake.