Preheat the oven to 350 degrees. F. Grease a 9-inch springform pan with butter or non-stick cooking spray.
Beat together the butter, both sugars, and peanut butter until light and fluffy. Add the eggs and vanilla and beat until well combined. In a small bowl, combine the flour, baking soda, and salt, until evenly incorporated. Add the dry ingredients to the standing mixer and mix on low until combined. Stir in the chocolate chips. Transfer the batter to the prepared springform pan.
Bake until a knife inserted in the center comes out clean, approximately 24-26 minutes. Allow to cool at room temperature.
Make the Peanut Butter Frosting. In a standing mixer fitted with the paddle attachment beat together the butter and peanut butter until light and fluffy. Add 1 and 1/4 cups confectioners’ sugar to the bowl and mix in, slowly at first until incorporated, then increasing the mixer speed to high. Add up to 1/4 cup more of confectioners' sugar, if needed to bring the frosting to a spreadable consistency. Frost the tops and sides of the cooled cookie cake.
Make the Dark Chocolate Ganache. Over low heat in a small saucepan, melt the dark chocolate. Once melted, remove the pan from the heat and whisk in the whipping cream. Put the ganache in the refrigerator for about 15 minutes to chill it (this will firm it up, making it easy to pipe onto the cake.) Transfer ganache to an icing bag fitted with a wide frosting tip (I just use a plastic bag with one corner cut off). Gently pipe 12 even dots of chocolate frosting around the edge of the frosted cake.
Cut each of the Reese's Peanut Butter Cups into 6 wedges. Top each dot of dark chocolate ganache with one wedge. Serve cake immediately or chill until ready to serve.