Either refrigerate or freeze the cake (once chilled or frozen, the cake can be easily cut into different shapes).
Prepare Buttercream Icing; set aside.
Use a long serrated knife, slice the cold cake in half or layers (depending on the size of your cake). You can measure the sides and mark them with toothpicks to help guide the knife; gently saw your way through. Cover cake layers with plastic wrap until you're ready to assemble them. Spread the filling (of your choice) between the layers. Place the top layer over the top of the filling.
Remove any crumbs from your work area.
Position the petit fours 2 inches apart on a thin mesh wire cake rack suspended over a baking pan (to catch drips).
Once your cake layers are filled, the simplest decorating technique for petits fours is to glaze or frost the top of the whole cake, and then cut it into shapes (squares, rectangles, rounds, or triangles, as desired). Each individual Petit Four cakes are usually about 1-inch square.
Allow frosted cakes to sit until they are dry, then remove the cakes from the rack, trimming any excess frosting away from the bottoms with a sharp knife.
Once your cakes are baked and cooled, they can be wrapped well and frozen for up to one month. Thaw the wrapped cakes at room temperature.
In a large mixing bowl, combine butter, milk, vanilla extract, rose water or rose extract, and powdered sugar; and mix at a low speed until smooth. If stiffer icing is needed or the weather is very warm, add a little extra sugar.
To create pastel colors, divide icing into a separate bowls for each desired color. Using a toothpick, add one drop of color at a time and mix until desired color is reached.
Chocolate Buttercream Icing: To create a chocolate icing, add 2 tablespoons of dark cocoa to the above recipe.
* Use vegetable shortening when pure white icing is needed.
Reva's Rose Petit Four Cake Recipe: https://whatscookingamerica.net/elleneaston/petitfourcake.htm