Heat oven to 350 degees F. Coat a 10-inch round springform pan with nonstick cooking spray. Line bottom of pan with waxed paper; coat paper.
Whisk flour and baking powder together. In bowl, beat whites until soft peaks form. Slowly add 1/4 cup of the sugar, until stiff peaks form.
In a separate bowl, beat yolks with remaining 1 cup sugar and vanilla until thickened, about 3 minutes. Fold in whites, then flour mixture. Pour batter into pan. Bake at 350 degrees F for 38 minutes or until top springs back when pressed. Cool in pan on rack 15 minutes.
In bowl, whisk both milks and heavy cream. With a wooden skewer, poke holes all over cake, 1 inch apart.
Place cake (in pan) on rack over a rimmed pan. Slowly pour soaking liquid on top, letting it seep into holes. Refrigerate overnight.
Place egg whites, sugar, water and cream of tartar in a metal bowl. Place over a pan half filled with gently simmering water. Beat until mixture registers 160 degrees F on an instant-read thermometer, about 7 minutes.
Remove side of pan; invert cake onto platter. Remove pan bottom and waxed paper. Frost cake and serve. Makes 20 servings.