• Preheat the oven to 180C /350F and line a cookie sheet with parchment paper.
• In a medium bowl, with an electric mixer, cream the butter and sugar until light and fluffy. Add in the egg yolk, vanilla, baking powder and salt and beat together. Finally add the flour and mix into a crumbly dough. When pressed with your hands it should hold together easily.
• Form it into a mound and place it on a sheet of parchment paper. The warmth from your hands helps the butter in the dough soften slightly, making it easier to work with. Add a second sheet and roll the dough into a 1/2 cm (1/4 inch) thick rectangle. Use a 6cm (2.5 inch) round cookie cutter to cut the cookies. Any scraps can be re-rolled and cut again.
• Bake the cookies, spaced 2cm apart for around 12 minutes until just lightly golden brown. Let them cool in a single layer on a cooling rack.

Marshmallows and assembling

• Prepare two piping bags by placing one each in tall glass. Fold the top of the bags down to widen the opening. You can use large round piping tips, or if you’re using disposable bags, you can snip off the ends once they’re filled.
• In the bowl of a stand mixer, combine 60g (1/4 cup) cold water and gelatin. Let this sit while you make the sugar syrup.
• Combine the remaining 60g (1/4 cup) water, granulated sugar, and salt in a small saucepan. Stir gently over medium-high heat just until the sugar has dissolved. Then stop stirring, let it boil, and cook until the mixture reaches to around 242°F / 116°C - 245°F / 118°C.
• Add the whisk attachment to your stand mixer and turn it on low speed. Drizzle the hot sugar mixture into the gelatin mixture. Gradually increase to high speed and whisk for 7-9 minutes until the mixture is thick, fluffy, and tripled in volume. Add the vanilla extract near the end.
• Divide the marshmallow mixture over the piping bags. Take one and snip off the end (if you’re not using a piping tip). Pipe the marshmallow evenly over each cookie. You can do layers of marshmallow, but do one layer at a time over all the cookies so it has a minute or two to slightly set before adding on another. Continue with the remaining marshmallow.
• Let them sit for at least 40 minutes before dipping in chocolate.

Chocolate coating

• Add the chocolate to a microwave-proof bowl and melt it in 20-30-second increments in the microwave, stirring after each burst. When it’s nearly all melted, remove it and stir to combine and melt the rest.
• Dip each cookie into the melted chocolate; you can push it quite far into the chocolate to coat it. Let the dipped cookie drip upside down and give it a gentle shake to remove the excess chocolate. Then, place on a cooling rack and let them sit at room temperature for 3-4 hours for the chocolate and marshmallow to firm up.