Make the Cake:

1. Preheat oven to 350 degrees.  Grease and flour three 6-inch round cake pans and set aside.
2. Sift together the flour, baking powder and salt.  Set aside.
3. In the bowl of an electric mixer fitted with a paddle attachment, beat the butter until smooth.  Add in the sugar and mix for about 3 minutes on medium speed.
4. Turn the mixer down to medium-low and add in the vanilla, eggs, and egg yolk.  Stop the mixer and scrape down the bowl.
5. Working in three alternating batches, starting and ending with the dry ingredients, add the flour mixture and milk on low speed.  Fold in the crushed chocolate cookies until combined.
6. Evenly distribute the batter between the pans and bake for about 24 to 26 minutes, or until a toothpick inserted in the center of the cakes comes out clean.  Cool on a wire rack for 10 to 15 minutes before removing the cakes from their pans.

Make the Mint Filling:

1. Place the egg whites and sugar in the bowl of an electric mixer. Stir to combine.
2. Fill a medium saucepan with a few inches of water and place over medium heat.  Put the mixer bowl on the top of the saucepan to create a double-boiler. While whisking, heat until the egg mixture reaches about 160 degrees on a candy thermometer.
3. Once hot, carefully return the bowl back to the mixer fitted with the whisk attachment.  Beat on high for about 7 to 10 minutes, or until the outside of the bowl returns to room temperature and the meringue holds medium peaks.
4. Stop the mixer and swap out the whisk for paddle attachment. With the mixer on medium-low, add in the butter, and few tablespoons at a time.  Mix until silky smooth.
5. Add in the vanilla, mint and green gel food coloring, if desired.  Mix until smooth.

Assemble the Cake:

1. Once the cakes are cool, trim and level the tops so that they are flat.
2. Place one cake layer on a cake board or serving dish.  Spread on about 3/4 cup mint filling.  Top with the next layer of cake and repeat.
3. Crumb coat the cake with the mint filling, then refrigerate to set – about 10 minutes.
4. Stir in 1 1/4 to 1 1/2 cups crushed chocolate sandwich cookies into the remaining mint filling.
5. Frost the outside of the cake with the cookie-studded icing.

Make the Glaze:

1. Place the chocolate, cream, and corn syrup in a saucepan and heat over medium-high heat.  Being careful not to scorch the cream, heat until the cream begins to steam and the chocolate starts to melt.
2. Remove from the heat, whisk until smooth.  Let cool.
3. Once glaze is completely cool but still pourable, lightly drizzle over the top of the cake, letting it drip down the sides.
4. Once the glaze sets, decorate the top with any remaining buttercream and mini chocolate sandwich cookies.