Spray a 17- x 11-inch jellyroll pan with cooking spray and line with parchment. Place template under parchment on one side of the pan.
To make stars, beat egg white with a fork until frothy. Add sugar and flour; beat with fork to incorporate. Add butter and mix well with fork to form a paste. Divide batter and add colorings to achieve desired colors. Transfer batters to piping bags or zip-top bags fitted with a piping tip. Pipe stars onto parchment, using template as a guide. Place pan in freezer for 15 minutes, at least.
Meanwhile, preheat oven to 375°.
In a stand mixer, beat eggs for 5 minutes using the whisk attachment. Gradually add sugar and continue to beat 5 minutes, or until egg mixture is pale yellow. Mix in vanilla and yellow food coloring.
In another bowl, combine flour, (xanthan gum, if using) and baking powder. Fold flour mixture into egg mixture, being careful not to deflate batter. Fold in water.
Remove jellyroll pan from freezer. Remove template from pan. Pour batter over patter and remaining pan. Spread batter evenly over pan with a spatula. Bake at 375° for 8-10 minutes, being careful not to overbake, or until cake springs back when lightly touched.
Once you remove cake from oven, sprinkle with powdered sugar and place another piece of parchment on top of sugar. Top with another jellyroll pan and flip cake to remove. Pull off parchment from the decorated side, and flip again so that the decorated side is facing down. Roll cake from the side opposite of the decoration. Let sit, rolled, until cake is cool.
Meanwhile, whip cream and sugar to stiff peaks with an electric mixer.
When cake is completely cooled, unroll cake, discarding parchment. Spread whipped cream over cake to edges. Top with berries. Roll cake tightly again. Serve immediately or chill.