Sift together flour, powdered sugar and cocoa powder. Set aside.
Using a handheld mixer or the whisk attachment of a stand mixer, beat egg whites, salt and cream of tartar to soft peaks. With mixer on low, slowly add vanilla and granulated sugar. Increase speed and whisk to stiff peaks. Fold in sifted dry mixture gently until no streaks remain and all ingredients are incorporated well. Spread mixture evenly into prepared pan.
Bake for 35-40 minutes or until internal temperature reaches 190°F. Cool in pan, then invert onto a wire rack or serving tray.
Before serving, dust with powdered sugar. Serve with whipped cream and strawberries.