Preheat oven to 350ºF. Line baking sheets with parchment paper.
In a large bowl mix together on high speed peanut butter with both sugars until creamy and smooth, about 2-3 minutes. Add eggs and vanilla. Mix until combined. Add baking soda and salt. Mix again until combined.
Put about 1/4 cup of sugar into a bowl. Roll dough into balls, about 1 tablespoon for each cookie, and then roll around in bowl of sugar. Place dough balls on prepared cookie sheets, about 2 inches apart. Use a fork to smash dough balls, creating a criss cross.
Bake 8-10 minutes, being careful to not overbake. Let cookies sit on baking sheet for a few minutes before transferring them to wire racks to cool.
In a small saucepan heat cream over medium heat until hot and just starts to bubble. Remove from heat and pour in chocolate chips. Let sit for 1-2 minutes and then stir until smooth. Let cool to room temperature while you make marshmallow buttercream and allow cookies to cool.
Cream together butter, marshmallow fluff, and powdered sugar on high speed for 2 minutes. Add vanilla and salt, and then mix until very fluffy, about 1-2 minutes more.
Place heaping teaspoonful of chocolate ganache in center of half the cookies. Once all the chocolate has been divided up between half the cookies, use a small spatula to spread evenly.
On the other half of cookies place a dollop of marshmallow buttercream on each. Once the buttercream has been divided up, sandwich the two halves together, pressing slightly so buttercream comes to the edge.
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To freeze or transport easily wrap up each individual cookie sandwich in plastic wrap. Store wrapped cookie sandwiches in airtight container if not eating right away.Adapted from Two Peas & Their Pod