Follow these step-by-step instructions carefully to create perfectly baked and decorated Chocolate Spider Cookies. The process is divided into making the cookie base and then assembling your spooky spiders.

Part 1: Making the Chocolate Cookie Base

1. Preheat and Prepare: Begin by preheating your oven to 350°F (175°C). Line two large baking sheets with parchment paper. This prevents the cookies from sticking and makes for incredibly easy cleanup.
2. Combine Dry Ingredients: In a medium-sized mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt. Whisking is important as it aerates the flour and ensures all the dry ingredients are evenly distributed, which is crucial for a uniform cookie. Set this bowl aside.
3. Cream Butter and Sugars: In a separate, larger bowl (or the bowl of a stand mixer fitted with the paddle attachment), beat the softened unsalted butter, granulated sugar, and packed brown sugar together on medium-high speed. Continue beating for about 2-3 minutes, or until the mixture is light, pale, and fluffy. This process, known as creaming, incorporates air into the dough, which is essential for a tender cookie.
4. Add Wet Ingredients: Scrape down the sides of the bowl with a rubber spatula. Add the room-temperature egg and vanilla extract to the creamed butter and sugar mixture. Beat on medium speed until everything is well combined and the mixture is smooth.
5. Combine Wet and Dry: Reduce the mixer speed to low and gradually add the dry ingredient mixture to the wet ingredients. Mix only until the flour is just incorporated. Be careful not to overmix at this stage; overmixing develops the gluten in the flour, which can lead to tough, dense cookies instead of soft, chewy ones. The dough will be thick and slightly sticky.
6. Shape the Cookies: Using a medium cookie scoop (about 1.5 tablespoons), scoop out portions of dough and roll them into smooth balls between your palms. Place the dough balls about 2 inches apart on the prepared baking sheets to allow for spreading.
7. Create an Indent: Using the back of a teaspoon or your thumb, gently press down in the center of each dough ball to create a small, shallow well. This indent is where the chocolate truffle “body” of the spider will nestle after baking.
8. Bake: Place the baking sheets in the preheated oven and bake for 8-10 minutes. The cookies are done when the edges are set and the centers look soft and slightly underdone. It’s better to slightly underbake them to ensure they remain chewy once cooled.
9. Press in the “Body”: As soon as you remove the cookies from the oven, while they are still hot and pliable, gently press one chocolate truffle into the indent of each cookie. The heat from the cookie will slightly melt the bottom of the truffle, helping it adhere.
10. Cool Completely: Let the cookies cool on the baking sheet for 5-10 minutes before transferring them to a wire rack to cool completely. It is absolutely essential that the cookies are at room temperature before you begin decorating with the chocolate legs. If they are even slightly warm, the piped chocolate will melt and lose its shape.

Part 2: Decorating Your Spiders

1. Melt the Chocolate: Place the semi-sweet chocolate chips in a microwave-safe bowl. Microwave in 20-second intervals, stirring well after each interval, until the chocolate is completely melted and smooth. Be careful not to overheat it, or it can become grainy. Alternatively, you can melt the chocolate using a double boiler.
2. Prepare for Piping: Spoon the melted chocolate into a small piping bag fitted with a small round tip (like a Wilton #2 or #3), or into a small zip-top bag. If using a zip-top bag, simply snip off a tiny corner to pipe from.
3. Pipe the Legs: Carefully pipe four spider legs on each side of the chocolate truffle body, extending out onto the cookie. Aim for eight legs in total for each spider. You can make them straight or give them a little “knee” bend in the middle for a more realistic, creepy-crawly look. Take your time with this step.
4. Add the Eyes: While the piped chocolate for the legs is still wet, you can use a tiny dab of the remaining melted chocolate as “glue” to attach the candy eyeballs. Place two candy eyes onto the front of the chocolate truffle body. This is the step that truly brings your spiders to life!
5. Let it Set: Allow the cookies to sit at room temperature for at least an hour, or until the piped chocolate legs have completely hardened. To speed up the process, you can place the decorated cookies in the refrigerator for about 10-15 minutes. Once set, they are ready to be served and admired!