Preheat oven to 350 degrees F. Grease the bottom of a 9x13 inch baking pan with butter. Line bottom with parchment paper and brush more butter on top.

Sift together cake flour, powdered sugar, and salt three times.

Combine egg whites, cream of tartar, and lemon juice in the bowl of a stand mixer. Beat egg whites with whisk attachment until soft peaks form. Slowly add granulated sugar 2 tablespoons at a time while mixer is still running until all of the sugar has been added. Continue to beat egg whites until medium-stiff peaks form.

Fold in ⅓ of the dry ingredients. Add ¼ cup strawberry puree and another ⅓ dry ingredients, folding until just combined. Add remaining ¼ cup strawberry puree and remaining flour, folding just until combined. Gently pour cake batter into prepared baking dish and spread in an even layer.

Bake strawberry angel food cake for 20-25 minutes or until golden brown and a toothpick inserted in the center comes out clean. Remove from oven and let cool completely on a wire rack.

Make chocolate whipped cream. Whip together heavy cream with cocoa powder and powdered sugar until stiff peaks.

Use a 3 ½-inch biscuit cutter and cut 6 rounds from the cake. Slice each round in half horizantally to create two layers. Spread chocolate whipped cream on bottom layer followed by reserved strawberry syrup. Stack top layer and spread more whipped cream and strawberry syrup. Grate chocolate on top if desired and serve immediately.