Lightly grease an 8- x 8-inch collapsible pan with canola oil spray (or brush on using a pastry brush), then use a sifter to dust generously with powdered sugar, shaking off the excess. Set aside.
In the bowl of a stand mixer fitted with the whisk attachment, combine ¼ cup water and ¼ cup Irish Whiskey. Sprinkle the gelatin over the liquid and let it sit while you make the sugar syrup. Make sure all the gelatin gets wet (you don't want a dry gelatin "island" floating around).
In a 3-quart saucepan, heat the remaining ½ cup of water, sugar, corn syrup, and salt over medium-high heat and stir until the sugar is dissolved. Using a digital thermometer (or a candy thermometer) bring the syrup to a boil without stirring until it reaches 238°F (soft-ball stage). It might foam up, so lower the heat if necessary. It takes about 20 minutes to get to the proper temperature. Remove from heat.
Turn on the mixer to Low to blend the gelatin for a few seconds. With the mixer running, slowly pour the hot sugar syrup in a thin stream down the side of the bowl. Be careful…you don’t want to have any splashes. This is a good time to use a splash guard if you have one.
Once all the syrup has been added to the bowl, gradually turn the mixer up to High, and whip for 12 minutes. Don’t be tempted to whip for less time...you need a lot a air whipped into the mixture to achieve that lofty marshmallow texture. When properly whipped, the mixture will be thick, and will slowly slump back into the bowl when the beater is lifted.
Pour the mixture into the prepared pan. You can smooth the surface with a small offset spatula. Sift a generous layer of powdered sugar on top.
Allow the marshmallow to dry on the counter, uncovered, for at least 4 hours. You want it to pull away cleanly from the sides of the pan when you gently tug it with your fingers. If you have the time, allow the marshmallows to dry overnight.
When you’re ready to cut the marshmallows, open the sides of the collapsible pan. Use the guides on the pan to cut the block into 1-inch squares with a plastic pizza cutter (mine came with the pan), a regular pizza cutter, or a chef’s knife. If you’re using a metal cutter or knife, wipe it with canola oil and dust it with powdered sugar repeating as necessary to keep it from sticking to the marshmallow block as it’s being cut.
Cut the block lengthwise into 8 strips, then crosswise into 8 pieces, to form a total of 64 squares. (For larger marshmallows, cut lengthwise into 6 strips, then crosswise into 6 pieces, to form a total of 36 squares.) Dust the cut sides of the marshmallows with additional powdered sugar as you go All the sides should be well coated and not sticky. See the Recipe Notes for additional cutting ideas.
Eat these adult-only marshmallows out of hand, made into S’mores, baked into brownies, or put a couple in a cup of coffee for the best Irish coffee you’ve ever had. Enjoy!
Marshmallows can be stored layered between sheets of wax paper or parchment in an airtight container in a dry place at cool room temperature, for 1 month. They can be frozen for up to 3 months.
Use any type of Irish Whiskey that you like to drink. You can also substitute in another type of spirit, like scotch, bourbon, or whatever you prefer.You can adjust the amount of whiskey you use up or down to suit your taste as long as you adjust the amount of water by the same amount. If you want kid-friendly marshmallows, substitute water for the spirits. Flavor the marshmallows with 1 teaspoon of vanilla extract or ¼ teaspoon of another extract flavor after all the whipping is done.You can substitute honey, maple syrup, or golden syrup for the corn syrup. Each will leave its subtle flavor on the finished product, and you can use it to enhance the whiskey's flavor profile.If you don’t have an 8- x 8-inch collapsible pan, line an 8- x 8-inch baking pan with parchment paper, leaving a 2-inch overhang. Spray lightly with canola oil spray (or brush on using a pastry brush), then sift generously with powdered sugar. When you're ready to cut the marshmallow block, lift it from the pan using the parchment overhang and place on a cutting board dusted with powdered sugar.You can cut the block into larger squares or make fun shapes with cookie cutter dusted with powdered sugar.