Line a 9-inch square baking pan with parchment paper, going up evenly on all four sides. (You can also use two pieces of parchment and criss cross them in order the extend the parchment over all four sides. This will create a sling, as pictured).

Place graham crackers, sugar, cocoa powder, and salt in the bowl of a food processor fitted with the metal blade. Pulse several times until the crackers are fine crumbs. Add the melted butter and pulse again; the mixture should look like dark, wet sand. Remove the blade, and give the mix a couple stirs with a rubber scraper to make sure the butter is all incorporated.

Transfer the crust mixture into the prepared pan, and press into an even layer. Use a flat-bottomed measuring cup to compact the crumbs and smooth it out. Place in the freezer for 20 minutes to firm up.

Place the powdered sugar, butter, peppermint extract, and vanilla bean paste in a medium-sized mixing bowl. Add the boiling water, and using a whisk, mix everything together until smooth. It should be a thick paste that smoothes back together after you stop mixing. If it doesn't flatten out after about 20 seconds, add more boiling water, 1 teaspoon at a time, until it's the right consistency.

Remove the crust from the freezer. Pour the peppermint filling on top, and gently bang the pan on the counter to level it out. Place in the refrigerator for 30 minutes to set.

Place the chocolate and oil in a microwave-safe bowl. Microwave in 30 second increments, stirring well after each, until chocolate is melted and mixture is smooth.

Remove the pan from the refrigerator, and pour the melted chocolate on top. Tilt the pan even it out. Again, gently bang the pan on the counter to smooth out the chocolate topping and pop any bubbles.

Return the pan to the refrigerator, and chill for at least an hour or until completely set. Pull up on the parchment paper, removing the bars from the pan, and place on a cutting board. While still cold, cut into slices using a sharp knife. Eat right away or store in an airtight container in the refrigerator for up to five days.