Mix butter and brown and regular sugar in a mixing bowl until creamy. Add vanilla and salt; mix until combined.
Add in flour and mix until the dough starts to form a crumbly mixture.
Mix in evaporated milk until the dough starts to look like play-dough. Add water.
Mix in cocoa powder. The dough should look just like play-dough. Stir in chocolate chips.
Press the dough into a 10x10 (or 9x11) inch and about 2 inches deep pan and bake at 350 degrees for about 20 minutes.
While the cookie layer is in the oven, melt butter for the caramel layer in a saucepan on medium heat.
Add brown sugar and stir on medium-high heat for 2 minutes.
Add evaporated milk and whisk vigorously for about 3-5 minutes, or until thickened and bubbly.
Remove from heat, add vanilla. Stir in cereal. Pour onto the cookie layer and refrigerate for 30 minutes.
Chocolate Marshmallow Layer:
In a saucepan on low heat, melt the marshmallows with the corn syrup, stirring until completely melted. then add the chocolate chips and stir continuously just until they're melted, then immediately pour onto the caramel and cookie layers. Wet your hands and press the melted marshmallows and chocolate onto the cooled caramel layer.
Refrigerate for at least 3 hours before serving. Cut into bars and enjoy.