Pre-heat oven to 375 degrees F.
Using a stand mixer fitted with the paddle attachment, cream butter, brown sugar and 1/2 cup white sugar until blended, light and creamy, about 3 minutes.
Add egg, 1/2 cup peanut butter, milk and vanilla, and beat until well blended, 1-2 minutes.
In a medium sized bowl, mix flour, salt and baking soda. Slowly add flour mixture to butter and sugar, just until dough comes together. Don't over mix. For best results chill dough 30 minutes, or over-night.
Roll dough into walnut-sized balls, about 1 Tablespoon. Or weigh the dough and divide the measurement by 36 for uniform cookies. Cookie dough can also be frozen at this stage.
Roll each ball additional sugar, then place on cookie sheets lined with parchment or silpat.
Bake at 375 degrees F. for 10-12 minutes, until light golden brown, but still soft.
Remove from oven and immediately place a peanut butter cup in the center of each cookie, pressing down slightly. While chocolate is still warm, attach candy eyes.
In a small bowl, melt chocolate morsels in the microwave at 30 second intervals, until chocolate is melted. Pour chocolate into a plastic sandwich bag. Snip off a tiny corner and drip 8 legs on each spider cookie. Chill until chocolate sets.