Make the Cake:
Position a rack in the center of the oven and preheat to 350 degrees F. Line the bottom of a 131/2 X9 1/2 inch pan with baking parchment paper. Lightly butter the parchment.
In a medium bowl, sift together the flour, cocoa powder, baking powder, baking soda, salt and sugar. In another medium bowl, whisk together the buttermilk, eggs and vanilla just until combined. Set aside.
In a small, heavy saucepan over medium heat, melt the butter and stir in the coffee. Using a wire whip, whisk this into the buttermilk mixture and combine with the dry ingredients. The mixture should be pourable but thick enough to coat the back of a spoon. Carefully pour the mixture into the prepared pan. Bake the cake for 25 minutes. The cake should pull away slightly from the edges of the pan and a toothpick inserted in the center should come out clean. Cool to room temperature.
Make the Peanut Butter Crunch:
Melt the milk chocolate in the top of a double broiler. Add the peanut butter to the melted chocolate and whisk until smooth. Cool, about 20 minutes. Fold in the crushed cornflakes. Set aside.Make the Coffee Syrup:
In a small, heavy saucepan, bring the water and sugar to a boil. Stir with a wooden spoon until the sugar dissolves. Let cool to room temperature. In a small bowl, whisk together the brewed coffee, sugar/water mixture, and rum. Set aside.
Make the Peanut Butter Mousse:
Melt the milk chocolate in the top of a double broiler. Add the peanut butter to the melted milk chocolate and whisk until smooth.
In a separate medium saucepan, bring the milk to a boil and remove from the heat. In two additions, slowly pour the milk over the chocolate/peanut butter mixture, mixing well after each addition.
Transfer the mixture to a 4 1/2 quart bowl of a heavy-duty electric mixer. Using the wire whisk attachment, beat on high for 5 minutes or until creamy. Whip the cream until it reaches the soft-mound stage. With a rubber spatula, fold the cream into the chocolate/peanut butter mixture just until incorporated.
Make the Glaze:
Place the chocolate in a medium bowl. In a small saucepan, bring the cream and corn syrup to a gentle boil. Pour the hot cream over the chocolate and let the mixture stand for 1 minute to melt the chocolate. Gently whisk until smooth. Cover the surface of the glaze with plastic wrap and let it stand at room temperature until ready to glaze the cakes.
Using the 4-inch ring molds, cut out 6 circles from the chocolate cake. With a long serrated knife, horizontally slice each circle in half. Place one cake round into each ring mold. Brush the tops of the cakes with the coffee syrup.Spoon a heaping tablespoon of peanut butter crunch on top of each round and with the back of a slightly wet spoon, smooth down evenly.
Fill a pastry bag (no tip is needed) with the peanut butter mousse. Pipe the mousse halfway up each ring mold. Place the remaining cake circle tops over the mousse, cut side facing up. Using a pastry brush, brush each circle with coffee syrup.
With the pastry bag, pipe the remaining mousse, completely filling the ring molds. Using an offset metal spatula, scrape the excess mousse away, leaving a smooth surface. Refrigerate until completely chilled and set, 4 to 6 hours.
Remove the molds from the refrigerator. Gently heat the side of each mold using a portable hair dryer or a hot, damp towel. Carefully slide off each mold. Place the cakes on a wire rack set over a baking sheet. Place the baking sheet in the freezer for 15 minutes.
Remove the plastic wrap from the glaze and gently whisk until smooth. Remove the desserts from the freezer and pour the glaze over each cake to coat it completely. Garnish with chopped peanuts, if desired.