Whisk the egg yolks, granulated sugar, and salt together in a heat-proof bowl. (I use a small glass bowl or liquid measuring cup.) Set aside.

Bring 3/4 cup (180ml) heavy cream to a simmer in a medium saucepan over medium heat. (The remaining heavy cream is used in step 6.) Once simmering, lower the stove’s heat to low.

Whisking the egg yolk mixture constantly, pour in 1/4 cup of the warm heavy cream. Whisk together until combined. Now, whisking constantly, pour the egg yolk/cream mixture back into the saucepan. Whisking constantly, cook the mixture until smooth and thickened, about 3 minutes or until it registers 160°F on a candy/oil thermometer. (I usually don’t measure the temperature, but you can if desired.)

Remove from heat and stir in the vanilla extract. Set aside.

Melt chopped chocolate in a double boiler or the microwave. If using the microwave: place the chocolate in a medium heat-proof bowl. Melt in 20 second increments in the microwave, stirring after each increment until completely melted and smooth. Stir melted chocolate into the egg yolk mixture. Set aside.

Using a hand mixer or a stand mixer fitted with a whisk attachment, whip the remaining 1 and 1/4 cups (300ml) heavy cream on medium-high speed until medium peaks form, about 3-4 minutes. Medium peaks are between soft/loose peaks and stiff peaks. Pour in the chocolate/egg yolk mixture and using a spoon or rubber spatula, gently fold together. Avoid over-mixing which can deflate the mousse. Makes 4 cups chocolate mousse.

Spoon a heaping Tablespoon of graham cracker crumbs into the bottom of a 6-ounce (or larger) glass, custard cup, ramekin, parfait glass, bowl, etc. Spoon 1/3 cup chocolate mousse on top. Spoon another heaping Tablespoon of graham cracker crumbs on top, then another 1/3 cup of chocolate mousse. Repeat with 5 remaining parfaits. Each parfait has 2/3 cup of mousse and 2-3 Tablespoons of graham cracker crumbs.

Chill for at least 3 hours and up to 1 day. If chilling longer than 3 hours, cover each with plastic wrap or aluminum foil.

Place egg whites, sugar, and cream of tartar in a heatproof bowl. Set bowl over a saucepan filled with two inches of simmering water. Do not let it touch the water. (You can use a double boiler if you have one.) Whisk constantly until sugar is dissolved and mixture has thinned out, about 4 minutes. Remove from heat. Add the vanilla extract, then using a handheld or stand mixer fitted with a whisk attachment, beat on high speed for 5 minutes until stiff glossy peaks form.

Spoon marshmallow cream on top of each chilled mousse cup. If desired, use a large piping tip instead. I used Ateco round piping tip 808. Serve immediately or store in the refrigerator, uncovered for 1-2 hours. If desired, toast the marshmallow topping with a kitchen torch just before serving.

Cover and store leftover mousse cups in the refrigerator for up to 1 day.