Step 1: Preheat your oven to 375°F (190°C). Line a large baking sheet (approximately 15x10 inches) with aluminum foil, ensuring it hangs over the sides for easy removal. Spray the foil lightly with non-stick cooking spray.
Step 2: Arrange the saltine crackers in a single layer over the prepared baking sheet, covering the entire bottom as much as possible. Break crackers as needed to fill in any gaps.
Step 3: In a medium saucepan, combine 1 cup (2 sticks) unsalted butter and 1 cup packed light brown sugar. Bring the mixture to a boil over medium heat, stirring constantly.
Step 4: Once boiling, continue to boil for exactly 3 minutes, stirring occasionally. Do not skip this step or cut it short, as it's crucial for the toffee to set properly.
Step 5: Carefully pour the hot toffee mixture evenly over the layer of saltine crackers, ensuring all crackers are coated.
Step 6: Transfer the baking sheet to the preheated oven and bake for 5-7 minutes, or until the toffee is bubbly and slightly darker around the edges. Keep a close eye on it to prevent burning.
Step 7: Remove the baking sheet from the oven. Immediately sprinkle 1.5 cups of semi-sweet chocolate chips evenly over the hot toffee layer. Let them sit for 5 minutes to melt.
Step 8: After 5 minutes, use an offset spatula or the back of a spoon to gently spread the melted chocolate evenly over the entire surface.
Step 9: If desired, sprinkle with flaky sea salt or your favorite toppings (chopped nuts, sprinkles, etc.) while the chocolate is still wet. This adds another layer of flavor and visual appeal.
Step 10: Place the baking sheet in the refrigerator for at least 1-2 hours, or until the chocolate is completely set and firm.
Step 11: Once completely chilled and hardened, carefully lift the foil-lined crack from the baking sheet. Peel off the foil from the back.
Step 12: Break the Christmas Crack into irregular pieces. Store in an airtight container at room temperature for up to 1 week, or in the refrigerator for longer freshness. For long-term storage, you can even freeze it!