Heat a large saucepan over high heat for 3 minutes. Add the oil and the popcorn kernels, and cover the pan with a lid. Cook, shaking the pan frequently, until you begin to hear popping. Cook on high for 1 minute more, then lower the heat to medium and continue cooking, shaking the pan, until the popping begins to slow down.
Turn off the heat and pour the popcorn into a large bowl. Toss with the melted butter, salt and graham cracker crumbs. Add the marshmallows and toss again. Spread the mixture out on a parchment-lined baking sheet and drizzle with the chocolate. Toss gently to incorporate. Set aside until the chocolate hardens, about 1 hour. Store in an airtight container.