With a hand mixer, cream together the peanut butter, butter, white sugar and brown sugar until blended. Beat in the egg, milk, and vanilla.
Combine the flour, baking powder and salt in a separate bowl. Add flour mixture to the peanut butter mixture and mix together. Stir in the Reese's Pieces, saving a few to sprinkle on the top later.
Press the cookie dough into a well-greased 14" pizza pan until the dough is evenly distributed. At this point, sprinkle the remaining Reese's Pieces on top. Bake for about 11 minutes at 375 and then remove from oven. Sprinkle the marshmallows on top and return to the oven for about 5-6 minutes or until the marshmallows are very lightly browned. Cool completely before preparing the chocolate and orange drizzle.
To prepare the chocolate drizzle, place the chocolate chips and shortening in a small glass bowl and microwave for 30 seconds. Stir and microwave at 15-second intervals, stirring each time until completely melted and smooth.
To prepare the orange drizzle, stir together the powdered sugar with the water (or milk) in a small bowl until well blended and then add the food coloring, mixing until color is uniform.
Drizzle the chocolate and orange drizzles on top of the cookie. Drizzle caramel topping if desired. Sometimes I just use a spoon to drizzle, or you can also use place the drizzle mixtures into small sandwich bags, snipping a small corner at the bottom to squeeze the drizzle out of.