Lightly spray a 9×13-inch pan with nonstick spray and set aside.
In a large pot over low heat, melt the butter and salt.
Whisk in the cocoa powder and espresso powder until smooth. Cook for about 30 seconds to bloom the cocoa.
Add the mini marshmallows and stir continuously until completely melted and smooth.
Remove the pot from the heat. Add the chocolate chips and vanilla extract. Stir until the chocolate chips are melted and the mixture is smooth.
Add the Rice Krispies cereal and gently fold until evenly coated.
Transfer the mixture to the prepared pan and lightly press into an even layer. Do not pack the mixture tightly.
Allow the treats to cool for 1 hour before cutting into 16 squares.