In the bowl of your mixer, stir together sugar, flour, cocoa, baking powder, baking soda and salt. Add eggs, milk, oil and vanilla; beat on medium speed for 2 minutes. Stir in boiling water. Pour batter into prepared pans.
Bake for about 30 minutes, or until tops of cake spring back when lightly touched in the center. Let cool in pans for about 5 minutes, then remove to cool completely on wire racks. Once cakes are room temperature, wrap and chill (preferably overnight).
Place the metal bowl of your mixer, along with whisk attachment, in the freezer for 5 minutes. Remove from freezer and add heavy cream, vanilla, and powdered sugar. Beat on high until you have whipped cream that forms stiff peaks (keep an eye on it....it can turn to butter if you let it go too long). Fold in crushed cookies.
Level your chilled cakes, then split each layer in half. Fill each layer generously with cookies and cream filling. If possible, stick in the freezer for about an hour, so the filling will firm up and not make it difficult to frost.
Prepare frosting: In the bowl of your mixer, beat butter and vanilla on medium speed until smooth. With the mixer on low, add powdered sugar and cocoa powder slowly until just barely mixed in. Add water and beat on high speed for about 2 minutes. Frost cake and serve.