In a large bowl or the bowl of a stand mixer, beat together the butter and sugar until light and fluffy. Add the eggs one at a time, followed by the vanilla, and mix well. Add the flour, baking powder, and salt, and mix until a firm dough forms. Chill 30 minutes.
Preheat the oven to 400. Roll the dough into one inch balls and place on baking sheets lined with parchment or sprayed with cooking spray. Flatten each ball slightly with your hand or with a drinking glass to be about 1/3-1/2 inch thick. Bake 8-10 minutes or until the bottom edges are just slightly golden brown. Remove and let cool before decorating.
When the cookies have cooled, melt the chocolate according to the package directions. Scoop the chocolate into a piping bag or ziplock bag, and snip a small piece off the corner of the bag. Pipe a circle of chocolate in the center of each cookie, and four legs coming out from each side. Place a Reese's cup in the center of each cookie. Pipe a small amount of chocolate on the front of the Reese's cup, and attach two candy eyeballs. Let cool and harden before serving.