For the cake: Sift together l cup sugar, the flour, cocoa, baking powder, baking soda, and salt. Set aside.
In the large bowl of an electric mixer, with paddle or beaters, beat the egg yolks at high speed. Turn speed to low and pour in the oil, water and vanilla. Gradually add the sifted ingredients, and when almost incorporated, turn speed to medium, and beat until well combined. Remove bowl from machine.
In another clean large bowl, with whip or beaters, whip the 6 egg whites until soft peaks form. Start on medium speed and raise speed as peaks begin to form. Gradually pour in the remaining l/2 cup sugar and whip until whites are shiny and firm but not stiff. With a rubber spatula, fold l/4 of the whites into the chocolate mixture, then scrape the chocolate mixture back into the whites, quickly folding until completely incorporated.
Scrape the mixture into a greased and floured half-sheet pan and bake at 375 degrees F. until edges of the cake pull away from the pan and a tester, gently inserted into the cake, comes out clean.
For the cream: In the large bowl of an electric mixer, using paddle or beaters, beat the mascarpone until fluffy, scraping down the sides of the bowl and under the beaters as necessary.
Meanwhile, in a medium bowl, set over simmering water, melt the chocolate. Set aside.
In a large heatproof bowl, set over simmering water, whisk the egg yolks with l/4 cup sugar until sugar dissolved and mixture is hot to the touch. Remove from heat and scrape into the mascarpone. On medium speed, beat just until combined. Lower speed, add the melted chocolate, and mix well.
In a large bowl of the electric mixer, using the whip attachment, whip the egg whites to soft peaks. Add the remaining l/4 cup sugar and continue to beat until shiny and firm but not stiff. Fold into the mascarpone mixture. Stir in the Kahlua and rum. Refrigerate until needed.
For the soaking liquid: In a small pot, combine all of the ingredients, bring to a boil. Remove from the heat and let cool.
To assemble the Tiramisu: I like to use parfait or Pilsner glasses for this dessert. Place all of the glasses on a flat work surface. Cut out cake circles that will fit into the size glass your using. Using a ladle, place a small amount of Mascarpone cream in the glass, dip a cake circle in the soaking liquid (squeeze out any excess) and place on top of the cream. Ladle more cream on top of the circle, place another soaked circle of cake and finish with a layer of cream. Repeat this with all of the glasses. Refrigerate for about an hour or until set. Decorate with grated milk chocolate and sifted cocoa.