In a large mixing bowl, sift together the flour, baking powder, and salt. Set aside.
Using an electric mixer, cream the butter with sugar until pale, light, and fluffy.
Add the vanilla, and beat in the egg yolks one at a time, scraping the bowl once or twice.
Add the flour mixture alternately with the milk, ending with the dry ingredients. Set the batter aside.
Using an electric mixer, whisk the egg whites with the cream of tartar just until the whites are stiff enough to hold a slight peak.
Fold the egg whites into the batter, 1/3 at a time.
Spoon the batter into the prepared pan, and spread it evenly over the cranberries.
Bake until the top is browned and the cake pulls away slightly from edges of pan, 25 to 35 minutes. My oven tends to run cool, it took about 40 minutes more before my center was set. Make sure to test the cake with a toothpick or cake tester. The center should not be loose/jiggly.
Let the cake cool for 15 minutes before turning it onto a cake plate.
Serve with slightly sweetened whipped cream, flavored, if desired, with orange liqueur. I served my cake plain, and it was delicious.