Preheat the oven to 350ºF. Spray a loaf pan with nonstick spray. Set aside.
In a medium size bowl, toss together the flour, cocoa powder, baking soda, and salt. Set aside.
In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar on high speed until light and fluffy, about 2-3 minutes. Add the eggs one at a time, scraping down the sides of the bowl when necessary to incorporate.
Reduce the mixer speed to low, add the milk and vanilla, and continue to beat on low until everything is combined, again, scraping down the sides of the bowl when necessary.
With the mixer speed on low, slowly add the dry ingredients and mix until just combined. Stir in half of the crushed graham crackers (about 1/2 a cup) and 3/4 cup of the mini marshmallows with a spatula.
Pour half of the batter into the prepared pan. Layer the remaining 1/2 cup of crushed graham crackers on top, then top with the rest of the batter. Scatter the remaining 1/4 cup of mini marshmallows on top of the second layer of batter, then bake the loaf for 45-50 minutes, or until a toothpick inserted into the center comes out mostly clean. Allow to cool 15 minutes before removing from the pan and allowing to cool on a wire cooling rack until ready to serve. Loaf stays fresh covered tightly at room temperature up to 5 days. Loaf freezes well, up to 2 months. Thaw in the refrigerator.