First, sift together the flour, baking powder and cream of tartar. In a large bowl combine the eggs, sugar, water and vanilla extract. Use an electric mixer (or stand mixer) to beat on high for 4-5 minutes until the mixture is light and airy. Fold the flour mixture into the egg mixture.
Cover the bottom of a jelly roll baking pan (10×15 inch with a lip) with parchment paper. Lightly spray the paper and exposed inside edges of the pan with cooking spray. Pour the batter into the pan, use a rubber spatula to make sure the batter is evenly distributed.
Bake at 375°F for 12-15 minutes until the edges begin to brown and pull away from the pan.
Allow the cake to cool for 30 minutes then flip out onto a clean work surface covered with a little powdered sugar or wax paper. In the above photo you can see the bottom of my cake. Cut off the edges.
Use peppermint bark squares to measure and cut the cake into 27-30 squares. Slice each cake square down the center, so you can make a peppermint bark sandwich, if you will.
Now to assemble our petit squares. 1.) Add a little frosting (store bought or a favorite recipe of yours) to the bottom of each cake square. 2.) Place the peppermint bark on the frosting and press in place. 3.) Add a little more frosting on top of the bark and press the top piece of cake in place. This will help your petit fours hold together once we get to dipping them. 4.) Roll out about one pound of marzipan very thin. Place all your prepared squares close together, spray one side of the marzipan with a little water and then place the wet side on top of the cakes. Press in place. If you refrigerate this for at least an hour (or overnight) it will help it stick to the cakes better. 5.) Cut the squares apart. Use kitchen shears to trim any excess marzipan off the edges. 6.) Dip each square in your prepared poured fondant (recipe follows). If you are adding any kind of sprinkles do so while the fondant is still wet.
Combine all ingredients in a large glass bowl set over simmering water. Stir until the mixture is well combined and very thin. Hold petit fours by the top and bottom and dip each edge into the icing. Place on a cooling rack set over wax paper. Use a spoon to pour icing on the top of the cake, so the entire square is covered (except the bottoms). Continue until you have covered all the squares. If you plan to add pipped frosting wait until the frosting dries (2-3 hours or overnight).