Line a 9X5 loaf pan with parchment paper or lightly spray it with cooking spray. Place the loaf pan in the freezer to chill while you prepare the ice cream base.
Add the heavy cream to a medium-sized bowl, along with 1 cup of just the cereal (not the marshmallows). Stir well to combine, then cover and allow to infuse in the fridge for 30 minutes.
Once the cereal has infused the heavy cream for 30 minutes, place a fine mesh strainer over a large bowl and pour the mixture through. The cereal will catch in the strainer, with the heavy cream collecting in the bowl. Discard (or enjoy) the soaked cereal.
Add the sweetened condensed milk and vanilla extract to a large bowl and stir to combine. Set aside.
Add the infused heavy cream to the bowl of a stand mixer fitted with the whisk attachment (or using a large bowl and an electric hand mixer). Mix the heavy cream on medium-high speed until stiff peaks form.
Add the whipped cream to the bowl of sweetened condensed milk and vanilla, and gently fold them together.
Add the marshmallows to the ice cream base, and fold them in very gently.
Pour the ice cream base into the prepared loaf pan and cover it with plastic wrap. Top it with additional marshmallows if desired.
Freeze for at least 4 hours before serving. 8-12 hours yields the best results.