Preheat oven to 350. Grease two 9-inch round cake pans.
In a large bowl, whisk flour, baking powder, and salt.
In another large bowl, beat egg whites and cream of tartar on medium speed until foamy and light in color. Turn beater speed to high and slowly add 1/3 cup of sugar. Continue to beat on high until soft peaks form.
In a third large bowl, beat butter, remaining cup of sugar, and vanilla extract until well blended. Turn mixer to low speed and add flour mixture slowly, alternating with champagne and heavy cream. Scrape bowl between additions and beat until combined. Turn speed to high and continue to beat about 30 seconds until thick and creamy.
Fold egg white mixture into batter slowly and gently, just until white streaks disappear. Pour batter into pans and place in oven for 25 minutes or until a toothpick inserted near the middle comes out clean. Cool cakes completely on a rack before removing from pans.
Fill with Strawberry Champagne Custard and ice with sweetened whipped cream - slightly flavored with strawberry juice or champagne if desired.