In a big bowl, using a mixer beat the butter, oil and the 1 1/4 C of sugar together for 3 minutes until the mixture is fluffy and its color turns lighter.
Beat in the egg yolks, one at a time, beating after each addition, and then beat in the vanilla.
In another bowl, sift the flour, baking powder, baking soda and salt.
In a different bowl place the cocoa powder, the semisweet chocolate, and the instat coffee. Add in the boiling water and mix until a not-so-thick paste forms.
Incorporate this chocolate paste to the butter mixture and beat until it is all completely incorporated.
Add in the flour mixture in three different additions, alternating with the buttermilk, starting and finishing with the flour. Beat until it is all incorporated.
In a completely dry and clean bowl beat, with a hand mixer, the egg whites with the 1/4 C of sugar until soft peaks form. Incorporate this to the batter in three additions using a rubber spatula, carefully and from the outside in.
Pour this batter into two different previously greased and floured circular cake pans (about 9 inches -23cm- in diameter) and bake at 350° F (180° C) for about 40 minutes or until a wooden stick comes out clean when inserted.
When the cakes are ready, take them out of the oven and let them cool for 10 minutes, then turn them out and place them on cooling racks for them to entirely cool down.
For the filling:
In a big bowl, using a mixer beat the cream cheese and butter for 2 minutes until the mixture is soft and fluffy.
Add in the dulce de leche, vanilla and salt, and beat until fully incorporated.
In a small and microwave safe bowl mix the marshmallows with the vegetable oil making sure to coat each one of the marshmallows with the oil (you can add some more oil if you feel like you need it). Put it in the microwave for 10 seconds, then mix it using a fork for the marshmallows to slowly melt; repeat this step once or twice more until the marshmallows melt into a sticky cream. Immediately add this cream to the cream cheese mixture and beat until fully incorporated.
Add in the condensed milk and beat for 1 min. until you get a soft creamy mixture.
Lastly, add the whipping cream and beat for 20 seconds, no more than that, just for it to incorporate, otherwise the mixture will curd.
Place all the mixture in a piping bag, with no piping tip (since it won´t be necessary) and put it in the fridge for 1 hour or until needed.
For the frosting:
In a small pot, mix the condensed milk, cocoa powder, egg yolk and salt. Cook it to medium heat for 3 minutes, stiring nonstop, until the mixture thickens a little.
Remove from the heat and stir in the chocolate chips and vanilla.
Once the mixture is no longer as hot, using a mixer, beat in the butter little by little until a soft frosting forms.
Cover it up with plastic wrap and put it in the fridge until neeeded.
To assemble:
Take the filling and the frosting out of the fridge.
With a really sharpen knife, cut each cake in half to get 4 layers of cake.
Place each one of the pieces on a big plate which will be where it will be presented. With the piping bag add enough filling, spread it all over the cake surface, and then place evenly one of the chopped snickers bar pieces. Repeat this step twice more and then finish with the last layer of cake.
Cover the cake with a lid and fridge it for 30 min. for it to be easier to frost.
Frost the cake the way you prefer until is entirely covered and then decorate with the chopped salted peanuts.