Spray the pot of a 4-quart slow cooker or CrockPot with pan spray and set aside. Cut the 1 (16.3-ounce) can refrigerated biscuits into quarters. Add the biscuits to a large ziplock bag along with ⅓ cup granulated sugar and 3 tablespoons ground cinnamon. Close the bag tightly, allowing room for air, and shake until all the biscuits are fully coated in the sugar. Add the coated biscuits to the prepared CrockPot. I sprinkled a small handful of the leftover cinnamon sugar over the biscuits and discarded the rest. Heat ½ cup unsalted butter, ½ cup brown sugar, and 1 large pinch salt in the microwave in 30-second increments until all the butter is melted and the sugar is mostly dissolved. Pour the brown sugar mixture over the biscuits. Cover and cook on LOW for 2-3 hours or until the pieces around the edge are starting to brown and the center reaches an internal temperature of 190 degrees Fahrenheit. Whisk the 1 cup powdered sugar, 3 tablespoons milk, and ½ teaspoon vanilla extract together in a small bowl and drizzle over the top of the monkey bread before serving.
