### Instructions
#### For the Chocolate Base:
1. Preheat your oven to 350°F (175°C). Line a 9×13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment.
2. In a large bowl, cream together the softened butter, granulated sugar, and light brown sugar until light and fluffy.
3. Beat in the egg and vanilla extract until well combined.
4. In a separate medium bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt.
5. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
6. Fold in the 1/2 cup semi-sweet chocolate chips.
7. Press the chocolate base batter evenly into the prepared baking pan.
8. Bake for 15-18 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs. Do not overbake as it will continue baking with the topping.
9. Remove from the oven and set aside to cool slightly while you prepare the fudgy topping.
#### For the Fudgy Topping:
1. In a medium saucepan over medium heat, melt the 1/2 cup butter.
2. Stir in the granulated sugar, light brown sugar, and evaporated milk. Bring the mixture to a gentle boil, stirring constantly. Continue to boil for 1 minute, stirring.
3. Remove the saucepan from the heat. Stir in the vanilla extract and salt.
4. Immediately add the 2 cups semi-sweet chocolate chips and the optional 1 cup milk chocolate chips to the hot mixture. Let sit for 2-3 minutes to allow the chocolate to melt, then stir vigorously until the mixture is smooth and glossy.
5. Pour the warm fudgy topping evenly over the slightly cooled chocolate base.
6. Gently spread the fudge to the edges of the pan.
7. If desired, sprinkle with flaky sea salt, chopped nuts, or additional chocolate shavings immediately.
8. Refrigerate for at least 2-3 hours, or until the fudgy topping is completely set. For best results, chill overnight.
9. Once thoroughly chilled, use the parchment paper overhangs to carefully lift the entire slab of bars out of the pan.
10. Cut into squares or rectangles using a sharp, warm knife (run under hot water and wipe dry between cuts for clean slices).
### Short Tips
* **Don’t Rush the Chill:** The chilling time is crucial for the fudgy topping to set properly and for the bars to cut cleanly. Patience is key!
* **Warm Knife, Clean Cuts:** For perfectly neat squares, dip your knife in hot water and wipe it clean between each cut. This prevents the fudge from sticking and dragging.
* **Prevent Sticking:** Don’t skip the parchment paper! It makes lifting the bars out of the pan a breeze.
* **Storage Savvy:** Store these bars in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to a week. They can also be frozen for up to 2 months. Thaw frozen bars in the refrigerator before serving.
* **Customize Your Crunch:** Feel free to swap out the chocolate chips in the base or topping for your favorite mix-ins like chopped toffee bits, mini marshmallows, or different types of chocolate.
