Prepare the candies: Unwrap and freeze the Reese’s cups for at least 15 minutes before baking. This keeps them firm when placed on the warm cookies.
Preheat the oven: Set your oven to 375°F (190°C). Line baking sheets with parchment paper or silicone mats.
Cream the base: In a large bowl, beat together the shortening, peanut butter, brown sugar, and granulated sugar until fluffy — about 2–3 minutes. The mixture should look creamy and pale.
Add wet ingredients: Mix in the egg, milk, and vanilla until fully combined.
Combine the dry ingredients: In a separate bowl, whisk the flour, baking soda, and salt. Gradually add to the wet mixture, stirring until just incorporated. Don’t overmix.
Form the cookies: Scoop the dough into 1-inch balls. Roll each in granulated sugar, then place 2 inches apart on the baking sheet.
Bake: Bake for 8–10 minutes, or until the edges are golden and the tops just start to crack.
Add the “spider bodies”: Remove the cookies from the oven and immediately press a frozen Reese’s cup into the center of each. Let the cookies rest on the baking sheet for 5 minutes before transferring to a wire rack.
Decorate: Melt the chocolate chips in a microwave-safe bowl in 20-second bursts, stirring until smooth. Use a piping bag or zip-top bag with the corner snipped off to pipe 4 legs on each side of the Reese’s cup. Attach candy eyes with a dab of melted chocolate.
Cool and serve: Let the chocolate set completely before serving. Store cookies in an airtight container at room temperature for up to 4 days.