Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper and set aside.
Place the softened butter and peanut butter in the mixer and beat until fluffy. Then add both sugar and beat again until fluffy. Scrape the bowl and beat in the vanilla and egg.
In a separate bowl mix the flour, baking powder, baking soda and salt together. Turn the mixer on low to slowly incorporate the flour mixture.
Roll the cookie dough into twenty-four 1 1/2 tablespoon-sized balls and spread 12 out on each cookie sheet. Bake for 10 minutes. Remove from the oven. Quickly use a pestle to press a cavity in the middle of each cookie. Then place back in the oven and bake another 6-10 minutes, until golden.
Meanwhile, unwrap all the chocolate truffles. Once the cookies are out of the oven, cool until almost at room temperature (but not very warm) and place a truffle in the cavity of each cookie. If the cookies are too warm, the truffle will melt. If the cookies are cool, the truffle won't stick. (You can also wait until the cookies are completely cool, then glue the truffles in with chocolate to be safe.)
Melt the chocolate chips in the microwave in 30 second increments, stirring in between, until melted and smooth. Scoop the melted chocolate in a zip bag. Close the bag and snip off a small portion of one corner to create a hole. Pipe 8 legs on each cookie, starting from the truffle body down.
Then use the remaining melted chocolate to glue 2 eyes onto each truffle spider. Cool until the chocolate is firm and serve.