In a medium mixing bowl, sift together cocoa powder, flour and salt. Set aside.
Into a large bowl, cream together butter and sugar. Add eggs and vanilla and continue mixing. Add the dry ingredients a little at a time and mix on medium speed just until combined.
Spray pans with non-stick cooking spray. Place about 2 cups of dough into each pan leaving 1/4" from the top. Wrap leftover dough in plastic wrap and place in refrigerator for 20 minutes to firm up. Bake pans for 45 minutes until cookie is baked through and a toothpick inserted comes out clean.
To Make Top Decorations:
Dust work surface with flour and roll out the leftover cookie dough until 1/4" thick. Cut out triangles and letters. Roll pieces of dough into a rope to make the remaining cookie accents. Place cut out cookie accents on parchment paper lined baking sheet and refrigerate for 15 minutes. Bake at 325ºF for 6 to 8 minutes or until set.
To Make Filling:
In a large bowl, cream together butter, sugar, vanilla bean paste and milk using a handle mixer set to medium. Line an 8" round cake pan with parchment paper and fill pan with filling using a spatula to make even and smooth. Place in refrigerator for at least 1 hour to set.
To Assemble The Cookie:
Place one large cookie, wider side up on a plate. Remove filling from cake pan and place on top of the cookie. Flip the second large cookie over and place on top of the filling. To add the decorations, use chocolate frosting as the "glue" to attach the accents. Enjoy and serve with lots of milk!