Spray the inside of the slow-cooker with cooking spray. In a large bowl, beat the cake mix, water, eggs, ½ cup peanut butter, and softened butter with an electric mixer. Beat on low for 30 seconds then on medium speed for 2 minutes, scraping the bowl, as needed. Remove 2/3 cup of this batter to a medium bowl and stir in ½ cup chocolate syrup to make chocolate fudge batter. Spoon ½ of the peanut butter batter into the slow cooker followed by all of the chocolate fudge batter. Top with the remaining ½ of peanut butter batter. Swirl with a knife in a circular motion to mix just a bit. Cover and cook on HIGH for around 2 hours or until a toothpick inserted into the center comes out clean.
Turn off the cooker and remove the ceramic insert to a cooling rack. Cool 15 minutes. In a medium bowl, whisk the 3 tablespoons of peanut butter with the milk until smooth. Add powdered sugar and milk until smooth. If necessary, gradually add 1 tablespoon additional milk until the glaze is the desired consistency. Spread this glaze over the cake. Then drizzle with the 2 tablespoons of chocolate syrup. Sprinkle the cut candies over the top of the cake. Serve from the pot.
NOTE: When baking in a slow cooker, watch to see that no condensation forms inside the lid and drips onto your cake. If you see that happening, put a paper towel over the top of the cooker and hold it in place with the lid.