Combine the flour, baking soda and salt in small bowl. Beat the butter, granulated sugar, brown sugar and vanilla extract in large mixer bowl until creamy, about 3 to five minutes. Add the egg and egg yoke, beating well. Gradually beat in the flour mixture and the 2 teaspoons of water. Stir in the chocolate chips. Cover and place the dough in the fridge for 15 minutes. Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper or silpats.
When the oven is preheated drop rounded tablespoon of dough onto the prepared baking sheets.
Bake for 9 to 11 minutes and DO NOT over cook. If anything undercook. Cool completely before stuffing with ice cream.
While the cookies cool let the ice cream sit out on the counter to soften and then when the cookies are cool enough turn one cookie over and place one (for a thin ice cream layer) or two (for a thicker ice cream layer) scoops of the ice cream onto the bottom of the cookie. Place the cookies on a baking sheet and place in the freezer while you make the hot fudge sauce. I think these are best eaten the day you assemble them, but they can be kept in the freezer until you are ready to eat them.
To make the hot fudge sauce stir together cocoa powder, brown sugar, corn syrup, cream, salt and half of the chopped milk chocolate. Place in a small sauce pan and cook over medium heat, stirring, until chocolate is melted. Cook mixture at a low boil, stirring occasionally for 5 minutes. Remove pan from heat, add remaining chocolate, butter and vanilla and stir until smooth. Any leftover sauce can be refrigerated in an airtight container. Reheat over low heat, stirring or in the microwave.
Remove the cookies from the freezer and top with the warm hot fudge sauce. Place another cookie on top and press down a little to push everything out a little. Eat immediately.