Make the cookies: Preheat the oven to 350°F and line a baking sheet with parchment paper.
Add the cubed butter, sugar, and brown sugar to a mixing bowl or the bowl of the stand mixer fitted with the paddle attachment. Beat with a hand mixer or the stand mixer until creamed.
Add the milk and vanilla extract. Beat again until well combined.
Add the flour, cocoa powder, 9 crushed oreos, baking powder, baking soda, and salt. Mix until just combined.
Place the 4 crushed oreos into a small bowl.
Split the dough into five equal pieces (~4oz each) and roll into balls. Roll in the crushed oreos, then place on the baking sheet, leaving room in between each. Press the balls down slightly.
Bake for 12-14 minutes. Set aside to cool completely.
Make the frosting: Add the softened butter to a medium-sized mixing bowl or the bowl of the stand mixer fitted with the paddle attachment. Beat with a hand mixer or the stand mixer until fluffy.
Add the powdered sugar, cocoa powder, 1 tsp milk, and vanilla extract. Beat until smooth and creamy. If necessary, add more milk, 1/2 tsp at a time, until the frosting is firm but smooth. Place into a piping bag and cut off the tip of the bag to create a small opening. Set aside.
Make the streusel topping: Add the flour, cocoa powder, and brown sugar to a small mixing bowl and stir. Pour in the melted butter and stir until a thick, crumbly dough is formed. If the dough seems wet, stir in a bit more flour or cocoa powder.
Assemble the cookies: Swirl the frosting onto the cooled cookies. Top with some bits of streusel and oreo crumbs. Top each with a gummy worm. Serve!