Step 1: Preheat your oven to 350°F (175°C). Line a 13x9 inch baking sheet with parchment paper, extending over the sides for easy removal. Arrange the 20 saltine crackers in a single layer over the parchment paper.
Step 2: In a small saucepan, melt 1/2 cup (1 stick) unsalted butter with 1/2 cup packed light brown sugar over medium heat. Bring to a boil, then continue to boil for 3 minutes, stirring constantly. Remove from heat and immediately pour the hot toffee mixture evenly over the saltine crackers, spreading with a spatula if needed.
Step 3: Bake for 5-7 minutes, or until the toffee is bubbly and golden brown. Remove from oven and immediately sprinkle the 6 ounces of semisweet chocolate chips over the hot toffee. Let sit for 5 minutes to allow the chocolate to melt, then spread evenly with an offset spatula. Sprinkle with sea salt if desired.
Step 4: Let the 'Christmas Crack' cool completely at room temperature, or in the refrigerator for faster setting, until hardened. Once firm, break the toffee into small, irregular pieces (about 1/2 to 1 inch). Set aside.
Step 5: For the cookies, reduce oven temperature to 375°F (190°C). In a large bowl, using an electric mixer, cream together 1 cup (2 sticks) softened unsalted butter, 1 cup packed light brown sugar, and 1/3 cup granulated sugar until light and fluffy (about 2-3 minutes).
Step 6: Beat in the 2 large eggs one at a time, mixing well after each addition. Stir in the 2 teaspoons of pure vanilla extract.
Step 7: In a separate medium bowl, whisk together the 2 1/2 cups all-purpose flour and 1 teaspoon baking soda.
Step 8: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined and no streaks of flour remain. Do not overmix.
Step 9: Gently fold in about 1 1/2 to 2 cups of the broken Christmas Crack pieces. Reserve any remaining pieces for pressing into the tops of the cookies before baking if desired.
Step 10: Drop rounded tablespoons of cookie dough onto ungreased baking sheets, leaving about 2 inches between each cookie. If desired, press a few extra crack pieces into the tops of each dough ball.
Step 11: Bake for 10-12 minutes, or until the edges are lightly golden and the centers are just set. They may look slightly underdone in the middle, but they will continue to firm up as they cool.
Step 12: Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. Enjoy your irresistible Christmas Crack Cookies!