Preheat oven to 350F. Grease three 5-inch cake pans and line the bottoms with parchment paper.
To prepare chocolate cake, in a large bowl, whisk the flour, baking powder, baking soda and salt.
In another bowl, whisk the boiling water and cocoa powder until smooth.
Beat butter and sugar together on medium speed in a bowl of a standing mixer until light and fluffy. Beat in eggs, one at a time. Beat in buttermilk and vanilla extract.
Reduce the speed to low and beat in half of the flour mixture, followed by the chocolate mixture. Beat in the remaining flour mixture until just incorporated.
Divide the batter and pour into the prepared cake pans. Bake 15 to 20 minutes or until a toothpick inserted into the cake comes out clean. Let cool completely.
For the rice krispies layer, melt chocolate and butter in a heatproof bowl over a pot of simmering water. Stir the mixture using a wooden spoon until melted and smooth. Let cool to room temperature. Then stir in rice krispies until evenly coated.
Divide the mixture onto the three cake layers and spread out evenly across the tops, going right to the edges.
To prepare frosting, in a large heatproof bowl, beat sugar with a handheld electric mixer for a minute until foamy. Increase speed and add sugar slowly while beating. Continue beating until meringue becomes stiff and glossy.
To assemble the cake, place one chocolate cake layer on a serving plate, spread a thick layer of frosting on top.
Repeat stacking two more times.
Using a kitchen torch, lightly brown the top and sides of the frosting.
Leave it at room temperature and serve the cake in the same day.