Preheat oven to 350 degrees F. Line 2 standard cupcake pans with liners and set aside.
Combine ingredients in a small bowl and mix until combined, and small clumps form.
Divide mixture evenly between cupcake liners, about 1 Tablespoon per liner. Pack it down firmly with the bottom of a small glass.
Bake 5 minutes, them remove to cool.
In a large bowl, combine dry cake mix and espresso powder.
Add remaining ingredients and whisk for 2-3 minutes, until batter is combined and uniform. Be careful not to over mix.
Using a large ice cream scoop, add batter on top of graham cracker crust, filling cupcake liners about ¾ of the way full.
If desired, sprinkle more graham cracker crumbs on top of each cupcake, then place pans in oven.
Drop oven temperature to 325 degrees and bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean.
Remove from oven and allow to cool in pans for 5 minutes, then remove and place on cooling rack.
When cool, using a paring knife, cut a small, 1-inch deep cone of cake out of the center of each cupcake. Set aside.
PEEPS® Buttercream Frosting
In the bowl of a stand mixer with a paddle attachment, beat butter 3-4 minutes until creamy.
Add in the confectioner's sugar slowly, about ½ cup at a time, until smooth. Taste after each addition to gauge how sweet you want it. Remember, you'll be adding melted sugar-covered marshmallows to this!
Add salt and milk, beating on low until incorporated, then on high for 2-3 minutes until fluffy. If you're making multiple colors, divide your buttercream into separate bowls.
In a double boiler (or a metal bowl set over a pot of boiling water - be careful with this!), melt your PEEPS®, using a silicone spatula to help the process. I used 4 colors, and melted each color separately, but you may only use one color if you're looking for an easier time.
Working quickly, add your melted PEEPS® to a bowl of buttercream, and mix with a spatula until fully incorporated and smooth. If only making one color, simply add all of the melted PEEPS® in to your stand mixer and beat on medium-high speed until incorporated.
Fill a piping bag with your icings, striping each color separately inside to achieve a variegated color.
Pipe a small amount of icing inside the cut out cupcake centers, and proceed to frost each cupcake, re-striping the bag with your colored icing each time you fill it. The result is unpredictable and so spring-like!
Decorate with sprinkles and full-sized PEEPS®, and enjoy!