For the cake: Preheat the oven to 325 degrees F. Liberally spray an 18-by13-inch sheet pan with nonstick cooking spray and line it with parchment paper. Stack the lined sheet pan inside a second sheet pan, to avoid extra browning when baking.
In a mixing bowl, add the flour, baking powder and salt and whisk to combine. Set aside.
In a food processor, pulse together the almond paste, granulated sugar and honey. Pulse until well combined. Open the food processor and scrape the sides and bottom with a metal spoon to check that the almond paste is completely incorporated. Add the butter, 1 tablespoon at a time, pulsing to combine after each addition.
Add the eggs one by one, pulsing until each is completely incorporated. After the last addition, scrape down the bowl and then pulse until the batter is completely smooth and slightly fluffy. Pulse in the flour mixture until blended.
Spread the batter evenly in the prepared sheet pan. Bake until the cake is golden brown and springs back when gently poked, 15 to 20 minutes. Let cool completely.
While the cakes are cooling, color the Royal Icing in the colors of your choosing and add each color to a pastry bag fitted with a #2 tip. Pipe tiny flowers on a piece of parchment by making small closed spirals in one color, then add leaves to either side in the other color. Set aside to set up and harden, about 1 hour.
Remove the cake from the sheet pan and cut in half widthwise. Spread the jam on top of one half and top with the second half. Freeze for 10 minutes to set.
Use a serrated knife to cut the cake into 1-inch squares. Place onto a parchment-covered sheet pan and freeze for at least 1 hour.
For the coating icing: In the meantime, add the white chocolate chips to a heatproof bowl and set over a pot of simmering water (the bowl should sit above and not touch the water). Heat, stirring, until melted and smooth.
Sift the confectioners' sugar into a separate large bowl. Stir in the hot water and corn syrup until smooth. Stir in the melted chocolate. If the icing is too thick (it should be pourable), add more hot water 1 teaspoon at a time. Use while warm; you can reheat it in the microwave if necessary as you are working.
Put the little cakes on a wire rack fitted inside a sheet pan lined with parchment paper. Coat the cakes with the icing using a spoon and small offset spatula, or add icing to a pastry bag, snip the corner and use to coat each cake. Top each with a hardened royal icing decoration. Allow the icing to set up before transferring to a serving platter.
In the bowl of an electric mixer fit with a paddle attachment, add the confectioners' sugar and meringue powder and beat to combine. Beat in 4 tablespoons of warm water to combine. Raise the paddle head to check the consistency of the icing; the icing should be thick and smooth but not dripping from the paddle. Add more water until the desired consistency is reached. (You can add slightly more water to achieve a flooding consistency icing to completely cover cookies. Use a stiffer consistency to pipe decorations and details.)