Step 1: Melt the Chocolate and Butter

1. Combine Chocolate and Butter: In a heatproof bowl, combine the chopped semi-sweet chocolate and unsalted butter. The combination of semi-sweet chocolate and butter gives these cookies a rich, fudgy base.
2. Melt Together: Place the bowl over a pot of simmering water (double boiler) and stir until the chocolate and butter are fully melted and smooth. You can also melt them in the microwave in 20-second intervals, stirring in between each interval to prevent burning.
3. Cool Slightly: Remove the bowl from the heat and let the chocolate mixture cool for a few minutes. This prevents the heat from the melted chocolate from scrambling the eggs in the next step.

Step 2: Prepare the Cookie Dough

1. Combine the Sugars and Eggs: In a large mixing bowl, whisk together the granulated sugar, brown sugar, and eggs until the mixture is thick and pale, about 2-3 minutes. This helps give the cookies a crackly exterior.
2. Add Vanilla Extract: Stir in the vanilla extract, mixing until fully incorporated. The vanilla enhances the chocolate flavor and adds a subtle aroma to the cookies.
3. Add the Melted Chocolate Mixture: Pour the cooled chocolate and butter mixture into the sugar and egg mixture, folding gently until combined. Be careful not to overmix.
4. Combine the Dry Ingredients: In a separate bowl, whisk together the flour, sifted cocoa powder, baking powder, and salt. Sifting the cocoa powder ensures there are no lumps in the cookie dough.
5. Add Dry Ingredients to Wet: Gradually add the dry ingredients to the chocolate mixture, stirring until just combined. The dough will be thick and fudgy, similar to brownie batter.
6. Fold in Mix-Ins (Optional): If desired, fold in chocolate chips and/or chopped nuts to add extra texture and flavor.

Step 3: Chill the Dough

1. Chill the Dough: Cover the mixing bowl with plastic wrap and refrigerate the dough for about 30 minutes. Chilling helps the dough firm up, making it easier to scoop and helping the cookies hold their shape.
2. Preheat the Oven: While the dough chills, preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper or a silicone baking mat.

Step 4: Shape and Bake the Cookies

1. Scoop the Dough: Using a cookie scoop or tablespoon, scoop 1-2 tablespoons of dough for each cookie and place on the prepared baking sheet. Space the cookies about 2 inches apart to allow for spreading.
2. Bake the Cookies: Bake in the preheated oven for 10-12 minutes, or until the tops are set and cracked but the centers are still slightly soft. The cookies will continue to set as they cool, so avoid overbaking for the best fudgy texture.
3. Cool the Cookies: Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.