Grease an 8x8 glass baking dish with the butter and then sprinkle about 1 tablespoon of powdered sugar so it is covering the dish. In the bowl of a stand mixer, mix 1/3 cup of gelatin over 1 cup of cold water and set aside.
In a large saucepan, combine granulated sugar, corn syrup, salt, and water. Stir this mixture on medium-low heat until all the sugar has dissolved. Bring the mixture to a boil until it reaches 240°F. If you don't have a candy thermometer, cook it until the mixture turns into a pliable, gel-like ball when placed in cold water. (That is how I always test it. You will know your mixture is getting to the correct temperature because the bubbles start getting viscous and are slow to pop.)
Pour the syrup gently into the gelatin and begin by mixing on low speed for about 6-8 minutes. Once it starts to thicken, add in your peppermint essential oil and food coloring. Turn the speed up to high until it has doubled in size and becomes thick, about 10 minutes. Pour into the baking dish and allow it to set for 8 hours or overnight.
Once the marshmallow has set, dust a cutting board or pan with powdered sugar and gently pull the marshmallow mold out. Cut six rows with a sharp knife, about 1 1/2 inch wide, and then cut each row into four squares until you have 24 marshmallows. Dust all of the sticky sides in powdered sugar. Store in a cute jar or container and display them proudly on the counter because they are as beautiful as they are delicious.
In a small saucepan, heat your milk, vanilla, and cacao powder over medium heat. Stir with a whisk for about 5-6 minutes until warm. Stir in stevia and cinnamon.
Pour your piping hot chocolate into a mug, about 3/4 full. Put one or two peppermint marshmallows in, and drizzle with chocolate syrup and crushed peppermint candies for garnish if you so desire. This stuff is Christmas cheer in a cup, so savor every velvety sip and then go make a cup for someone you love!