Grease a 10-inch pie pan and set aside

To make your crust, in a bowl, combine your graham crackers, melted butter, and sugar. Stir until it looks like wet sand. Press firmly into your pie dish and use a flat-bottomed measuring cup or glass to pack in

Place your crust in the freezer for 15 minutes and preheat your oven to 350°F

Bake your crust for 15 minutes, remove it, and allow it to cool completely

Add your marshmallows and ½ cup of heavy cream to a small pot over low heat. Stir until smooth. Remove from the heat to cool for 5 minutes, then place in the fridge until cool (anywhere from 30 minutes to an hour)

In a large bowl, add the remaining 2 cups of heavy cream and your cooled marshmallow mixture. Beat until fluffy and firm peaks form. Pop in the fridge until ready to assemble

Place a handful of your blackberries into a bowl and with a fork or the bottom of a glass, mash them up. With your remaining blackberries, leave some whole and cut some in half along the stem

To assemble, spread your marshmallow whipped cream inside your graham cracker crust. Top with your mashed blackberries and use a knife to swirl them into just the top of your cream. Top with your remaining berries

Place in the fridge for at least 3 hours to set. Slice and enjoy!