If using instant yeast, add the bread flour, water, salt, instant yeast, sugar, and vanilla to a large bowl and mix until no dry flour remains. The dough will be sticky and shaggy, which is normal.
If using active dry yeast, first warm the water to 100 to 110 degrees F, stir in the yeast and a pinch of the sugar, and let it sit 5 to 10 minutes until foamy, then add it with the rest of the ingredients and mix.
Cover and let the dough rest for 30 to 60 minutes.
Add in Crushed Graham crackers, is using. Perform three sets of stretch and folds, spaced 30 minutes apart. With a wet hand, grab one edge of the dough, stretch it up, and fold it over itself. Rotate the bowl and repeat a few times for each set. After the last set, cover and let the dough rise for about 1 - 1.5 hour, until puffy and noticeably risen.
Line a 9x13 inch pan with parchment paper. Melt the butter and stir in the vanilla in one shallow bowl. In a second bowl, whisk together the crushed graham crackers, sugar, and cornstarch.
Lightly oil your hands. Pinch off a small ball of dough, dunk it fully in the vanilla butter, then roll it in the graham coating until covered. Place it in the pan. Repeat to make 24 balls in a 6 by 4 grid, all touching. Reserve any leftover coating for the top.
Cover and proof for about 1 hour, until the balls are puffy and have grown together.
Press a square of chocolate into the center of each ball, pushing it all the way down to the bottom. The dough springs up as it bakes, so a shallow piece will get pushed back to the surface.
Bake at 425 degrees F for 22 minutes.
Scatter mini marshmallows over the top in small clusters, two or three per ball, then sprinkle with the reserved graham coating. Bake for 3 more minutes, for 25 minutes total, until the marshmallows are puffed and golden. The internal temperature should read at least 190 degrees F.
Let cool for 5 to 10 minutes, then pull apart and serve warm.