Place the finely chopped chocolate into a medium size bowl. Set aside.

In a small saucepan over medium heat, combine the heavy cream and butter and heat to steam. Remove from heat and immediately pour over the bowl of chocolate. Allow to sit for a few minutes, then carefully stir mixture until all of the chocolate is melted. Allow to cool completely at room temperature before using.

In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, cream the butter until it is light and fluffy. Add the marshmallow creme and beat on high until smooth (approximately 2 minutes).

With the mixer on low, add the powdered sugar. Increase mixer speed to medium and beat until completely combined (approximately 1 minute).

Layer approximately 1/3 of the marshmallow buttercream evenly into the bottom of a medium size serving bowl. Layer half of the ganache on top of the buttercream. Repeat again, finishing with the last 1/3 of the marshmallow buttercream.

Gently stir the dip with a butter knife, allowing the layers to just combine. Top with mini-marshmallows, if using.

Serve dip with graham crackers (or your favorite crunchy dipper). Leftover dip can be stored in the refrigerator, covered tightly, up to 5 days. Allow to come to room temperature before serving.