Preheat oven to 350 degrees Fahrenheit. Grease an 8x8 baking pan with coconut oil, then line with 2 pieces of parchment, one laid in each direction.
In a food processor, pulse graham crackers until finely crumbed. Transfer to a medium bowl and add melted coconut oil and agave. Stir with a fork to combine, then use your hands until mixture is evenly combined. Add crust mixture to prepared pan, pressing firmly to create an even layer. Bake for 10 minutes. Place on a wire rack to cool.
Rinse out your food processor bowl and wipe clean. When your crust layer has baked and is cooling, add hot water and cocoa powder to food processor and let sit 5 minutes. Add melted chocolate, agave syrup, vanilla, coconut oil, and salt and process until mixture is very smooth and thick, about 30 seconds.
Pour chocolate mixture into pan over crust. Shake slightly so it settles evenly. Top with marshmallows, pressing down gently. Chill in refrigerator until chocolate layer is set, at least 1 hour. When ready to serve, set broiler on high. Place pan on middle rack and broil for 1 to 2 minutes, watching, until marshmallows start to brown. If needed, place pan in freezer for a couple minutes before cutting. To cut, remove bars from pan using parchment, cut into bars with a sharp knife.