Preheat oven to 325°F. Spray a large fluted tube (Bundt) cake pan thoroughly with nonstick spray and set aside.
Open the 48 oz tubes of Pillsbury biscuits and separate the biscuits. Cut each biscuit into quarters.
In a large resealable plastic bag, combine 1 cup granulated sugar and 2 tsp cinnamon. Seal the bag and shake to mix.
Add the biscuit quarters to the sugar–cinnamon bag, seal, and shake until all pieces are evenly coated.
Place the coated biscuit pieces in the prepared pan, arranging them to fill the pan and packing them together so there are no large gaps.
In a medium bowl, combine 1 cup melted butter and ¾ cup brown sugar; stir until smooth and combined. Pour this mixture evenly over the biscuits in the pan.
Bake for about 50 minutes, or until the top is golden brown and the center is no longer doughy. Remove from the oven and let the monkey bread cool in the pan for 10 minutes.
After 10 minutes, invert the pan onto a serving platter to release the monkey bread. If it does not release immediately, gently tap the pan and lift.
In a small bowl, whisk together 2 cups powdered sugar, 2 tbsp milk, ½ tsp vanilla extract, and a pinch of salt until you reach a thick glaze consistency. If the glaze is too thick, add a little more milk, 1/4 tsp at a time, until desired consistency.
Drizzle the glaze over the warm monkey bread and serve.