Preheat oven to 350°F.; Grease and flour baking pan (13x9x2-inches) or 2 round layer pans,(8 or 9x1.5 inches).
Sift all the dry ingredients together into a large mixer bowl. Although flour comes presifted, this step is designed to incorporate the soda and baking powder, salt, and spices into the flour and thus avoid salty lumps. Add remaining ingredients, cutting up the butter. Beat 1/2 minute on low speed, scraping the bowl. Beat for 3 minutes at high speed, occasionally scraping the bowl. Pour batter into prepared pan(s).
Bake at 350°F. for 45 minutes (oblong pan) or 40 minutes (rounds), or until a cake tester comes out clean. Cool on a cake rack for 15-20 minutes, while frosting cools. Frost the cake.
When the cake has about 20 minutes left to bake, prepare the frosting: Melt the butter in a saucepan over low heat; do not let it brown. Stir in the brown sugar and salt. Bring to a boil over medium heat, and boil hard for 2 minutes, stirring with a rubber scraper the whole time. Remove the pan from the heat. Add the warm milk in which you have dissolved the buttermilk powder. Return the pan to the heat and bring to a full boil again. Remove the pan from the heat and set aside to cool to lukewarm (about 20 minutes). Put the butter mixture and the confectioners' sugar into a mixer bowl and beat at high speed until very thick and smooth. If the frosting hardens too much to spread, beat in a few drops of milk. This is enough frosting for either one oblong or two round layers.
If preparing a layer cake with the two round cakes, spread frosting on top of one cake, stack the second cake on top, and then cover the layered cakes with the remaining frosting.
Allow the cake to come to room temperature before wrapping tightly. This cake keeps at room temperature for about 3 days and freezes well for 3 months, well wrapped.