1. Preheat the oven to 350℉. Line an 18x13x1 (or 17×12) sheet pan with parchment paper or greased foil, leaving an inch or two hanging over the edge.
2. Make the cookie: In a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, brown sugar, and vanilla until light and fluffy. Add the eggs, one at a time, beating well after each addition. (*If using the softer cookie adaptation described in the recipe notes, add the corn syrup with the eggs.)
3. In a medium bowl, mix the flour, salt, and baking soda. Half at a time, add the dry ingredients to the wet ingredients and beat until incorporated. Stir in the chocolate chips.
4. Dollop the batter onto the prepared sheet pan, covering as much of the pan as possible. Spread the dough into an even layer that covers the entire surface of the pan. (I like to use an offset spatula or even a dinner knife and hold onto an edge of the parchment to keep it from slipping.) The batter will just cover the pan completely but will not appear very thick; it will rise while baking.
5. Bake for 13-15 minutes or until just cooked through in the center and lightly golden on top. It should still feel soft to the touch; make sure to not overcook.
6. Cool completely in the pan. (Tip: Once cool, I like to refrigerate for easier handling.) Then cut the cookie in half crosswise to form two equal-sized rectangles. Freeze the cookie halves until firm, at least 30 minutes or up to several days. More tips: Make sure the cookie is on a flat surface when frozen. I use a piece of cardboard as a supportive base because it fits better than a baking sheet in my freezer. Place a piece of parchment, wax paper, or foil under the bottom layer (for easy transfer) and between the halves (so they don’t stick to each other). If freezing for more than a few hours, cover the cookies.
7. When ready to fill, remove the ice cream from the freezer to soften – but not too soft. Leave one cookie half in the freezer and place the other half, bottom side up, on a piece of parchment paper on a baking sheet or piece of sturdy cardboard.
8. Working quickly, spoon large scoops of ice cream all over the cookie. Spread into an even layer, making sure the ice cream reaches the edges of the cookie. (Tip: If at any point the ice cream becomes too melty, simply return to the freezer until firm enough to proceed.) Remove the other cookie half from the freezer (leaving it in the freezer avoids further melting of the ice cream) and place it on top of the ice cream, right side up, to form a giant sandwich. Press lightly, and then freeze until firm, at least 3 hours or overnight. (Pro tip: If you’d like really clean edges, when firm, soften a little extra ice cream and fill in any gaps along the sides. Alternatively, you could slice off the edges with a sharp knife.)
9. Cut into 12 or 16 individual sandwiches. (Tips: It’s helpful to rinse the knife in hot water—and quickly dry—in between cuts. Also, remember that you can put the bars back in the freezer if at any point the ice cream becomes too soft.)
10. Decorate if desired: Place mini chocolate chips, sprinkles, mini M&Ms, and/or chopped nuts in small bowls and press each side of the sandwiches into the toppings so they stick. Brush the excess topping from the sides of the sandwiches so only the ice cream is covered. Freeze until firm, and then serve or wrap individually with parchment or wax paper.
Storage: The chipwich bars will keep for at least 2 to 3 weeks in the freezer, much longer if they are well wrapped and stored in the bottom/back of the freezer, where the temperature tends to stay constant and discourage freezer burn.