Prepare the Pan: Line an 8x8-inch or 9x9-inch pan with parchment paper.
Melt Base Mixture: In a large pot over medium heat, melt butter and brown sugar together, stirring until smooth. Stir in vanilla and salt.
Add Marshmallows: Add 2 cups of mini marshmallows to the pot and stir until completely melted and combined.
Mix: Remove from heat and quickly fold in graham cracker pieces until well coated.
Press Into Pan: Gently press the mixture evenly into your prepared pan.
Add Chocolate: Sprinkle chocolate chips evenly over the top while the mixture is still warm. Let sit for a few minutes, then spread the melted chocolate into an even layer.
Top with Marshmallows: Sprinkle remaining 1 cup mini marshmallows over the top and gently press them in.
Chill: Refrigerate for about 1 hour or until set.
Toast (Optional): For a true s'mores vibe, broil the top for 1–2 minutes or use a kitchen torch to toast the marshmallows lightly before serving.