Grease a 9x9-inch baking pan with unsalted butter and line it with parchment paper.

In a large mixing bowl, measure out 5 cups of Rice Krispie cereal and set aside.

In a medium saucepan over medium heat, melt 3 tablespoons of unsalted butter until golden brown, about 3-5 minutes.

Reduce heat to medium-low and add 10 ounces of marshmallows, stirring until melted and smooth, about 2-3 minutes.

Mix in 2 tablespoons of espresso powder and 2 tablespoons of condensed milk to the melted marshmallow mixture.

Pour the warm marshmallow mixture over the Rice Krispie cereal and gently fold until evenly coated.

Spoon the coated cereal into the prepared baking dish and press down evenly.

Let cool for 5-10 minutes at room temperature, then cut into squares or bars.