Preheat oven to 350°F. Melt butter and brown sugar over low heat while stirring until sugar crystals have melted. Turn off heat and pour glaze into an 8 inch round cake pan. Place prunes on top of glaze in a spiral pattern. You may have some prunes leftover. Set pan aside. In a bowl, sift together flour, baking powder and salt. In a mixer, cream butter and sugar together. Add egg yolks and vanilla. With the mixer on low, add half the flour mixture and beat, then add the milk, and finish with the other half of the flour. Whip the egg whites and cream of tartar to soft peaks, fold into the batter, then pour the cake batter on top of the prunes and bake at 350°F for 1 hour or until toothpick inserted in center comes out clean. Cool for 20 minutes. To un-mold the cake, invert the pan onto a cake plate. Gently tap the bottom of the cake pan to release any prunes and slowly remove the cake pan. Slice and serve as is or with whipped cream.