In an electric mixer fitted with the paddle attachment, mix the butter and sugar together until just incorporated. Do not over-mix at this stage, or the cookies may spread while baking.
Add the egg and vanilla extract. Mix again on low speed, stopping to scrape down the sides of the bowl intermittently as needed.
In a medium bowl, whisk together the flour and salt. Add to the butter and egg mixture. Mix on low speed until a dough is formed and there are no longer any streaks of butter in the mixing bowl. The dough will often clump around the paddle attachment while being mixed. This is normal and a good sign that your dough is the right consistency. If your mixture does not come together and is crumbly, add ice cold water 1 tbsp. at a time until the dough clumps. Roll the dough to 1/2-inch thickness between sheets of parchment paper and chill until ready for use, at least 30 minutes.
Preheat the oven to 350°F. Line two baking sheets with parchment paper.
Cut the dough into four 5×7-inch rectangles (you can use a post card for a template!), re-rolling dough scraps if needed to cut exactly four pieces. Bake for 20 minutes, or until the cookies are fragrant and golden brown on the edges. Transfer the cookies to a wire rack to cool completely.
For the decors
Combine the candy in a microwave-safe bowl and heat at 1 minute intervals until the candy can be stirred smooth. Transfer the candy to a piping bag and snip the end. Pipe candy into the molds and tap the molds on a work surface to remove air bubbles. Transfer molds to a baking sheet and freeze for 10 minutes. Gently turn out the candy frames onto a piece of parchment paper. Attach the frames to the cooled cookies using dots of buttercream.
Using the center of the frame mold as a guide, create an oval paper template. Use the template and food color marker to create oval shapes on the rice papers. Cut out the ovals using an x-acto knife or kitchen-dedicated scissors.
Use a soft bristle brush to coat a small area of the rubber stamp with brown food color. Stamp the lower third portion of the rice paper ovals using the stamp. (Re-ink with food color as needed). Set aside to dry, about 20 minutes. Place tiny dots of piping gel around the inner edge of the ovals on the candy frames. Place the oval rice papers inside the ovals and press lightly to adhere. Let dry while you cast the fondant ferns and fiddle heads.
Lightly dust the fern mold with powdered sugar and tap out any excess. Knead together the green fondant and gum pate. Press the mixture into the molds until overflowing with fondant, and then trim the excess away using a knife (see video for visual). Flip the mold over and bend to release the fondant pieces. Let dry until firm, about 1-2 hours depending on humidity.
Attach the fondant decors to the rice paper using dots of piping gel or corn syrup. Brush brown petal dust or cocoa powder around the edge of the rice paper to create an aged appearance. Use a dry brush and tap away the excess before applying to the paper.
Dip a dry brush into the gold luster dust and apply to the green ornate frames. Apply a little at a time, so that some of the green still shows through the gold.
Let the cookies stand for 1 hour to set completely before packaging or transferring to bakery boxes.