Preheat the oven to 350 degrees F. Line a 18x12-inch rimmed baking sheet (half-sheet size) with parchment, and spray with nonstick cooking spray. Sift together the cake flour and salt.
Combine the butter, cream cheese, and sugar in the bowl of a large stand mixer. Beat them together on medium-high speed until very light and fluffy, 3-4 minutes. Reduce the speed to medium and add the eggs one at a time, beating for 1 minute after each addition. Add the vanilla extract. Finally, turn the speed to low and add the cake flour, mixing until just a few streaks of flour remain.
Stop the mixer and finish mixing by hand, scraping the bottom and sides of the bowl and stirring well. Scrape the batter into the prepared pan and smooth it into an even layer.
Bake the cake for about 20 minutes, until it’s golden brown and it springs back when touched lightly with the fingertips. Let it cool completely at room temperature. The cake can be made several days ahead of time; wrap it well in cling-wrap if you’re making it in advance.
To Assemble the Petit Fours:
Make a vanilla simple syrup by combining 1 cup of water and the sugar in a small saucepan over medium heat. Stir until the sugar dissolves, then bring the mixture to a boil. Once it boils, remove the pan from the heat and stir in the vanilla extract. Let it cool to room temperature while you prepare the cake. Have your room temperature frosting and room temperature raspberry jam ready.
Flip the cake upside-down onto a cutting board and peel off the piece of parchment. Cut the cake into six rectangles by cutting the cake across the middle lengthwise, then cutting it into thirds widthwise. Use a long serrated knife to carefully cut each rectangle into two thin layers (like you would cut a cake into layers) so that you’re left with 12 thin rectangles.
Brush one rectangle with the simple syrup mixture. Dollop on a spoonful of raspberry jam, and spread it in a thin layer over the cake. You want enough jam so that the flavor comes through, but not so much that the cake is unstable. Top the jam with another cake slice, and brush it with more simple syrup. Top this cake layer with a thin layer of frosting, about the same height as one of the cake layers. Brush one more cake layer with simple syrup, then place it, syrup side-down, on top of the buttercream. You should now have a 3-tiered rectangle of cake. Set it aside and repeat this process with the remaining cake layers, until you have four rectangles of assembled cake.
If you're using marzipan, divide the marzipan in four pieces and roll each into a very thin layer—it should be so thin it is almost translucent. Brush the top of one of the rectangles with a bit of simple syrup, and press the marzipan on top. Trim the edges so the marzipan is flush with the sides of the cake, and repeat until all of the cake rectangles are covered with marzipan. Refrigerate the tray to firm up the cake rectangles, for about 45 minutes.
Once firm, use a round cutter to cut small circles out of the cake, about 1 – 1 1/2 inches across. If the cake layers are taller than the cutter, try pressing the cutter down until it is flush with the top of the cake, then using a paring knife to trim around the bottom—this prevents the top of your petit fours from getting dented or smashed when you try to press the cutter to the bottom of the cake. Alternately, you can cut them into small squares instead with a large sharp knife. Place the petit fours on a baking sheet and freeze them until firm, about 1-2 hours. The petit fours should be very hard before you dip them.
Warm up your fondant in the microwave until it is very warm and very fluid—it should readily pour from a spoon. Warm it in 30-second intervals, stirring after every 30 seconds to prevent it from overheating. You will need to warm up the fondant fairly frequently during the dipping, so it’s easiest if you work near the microwave.
Take the tray of cake from the freezer. Press a wooden barbeque skewer through the bottom of one of the pieces, until it is firmly in the middle but not poking through the top. Dip the top of the cake into the fondant at a 45-degree angle and swirl it around. Use a spoon to pour fondant on the sides of the cake as you twirl it. Remove it from the fondant, let the excess drip back into the bowl, and use a spoon to touch up any areas that were missing fondant. Take a fork and put it under the piece of cake, twist the skewer to remove it so the cake is resting on the fork, then gently set the cake onto a wire cooling rack to set. (Refer to the video linked in the Notes section for a visual demonstration of this step.) Repeat until all of the pieces of cake are dipped. If the fondant gets too thick, re-warm it in brief intervals in the microwave. Once the fondant is set, use a sharp knife to carefully loosen them from the wire rack. If desired, you can top them with a pinch of disco dust or sprinkles when the fondant is still wet, or press a bit of edible gold leaf on top of the fondant once it is dry.