This is a high, light-textured chocolate cake with a bit of rum flavor, nice for spring. A nice cake for a wedding shower or for Mom on Mother's Day -- if you wish, sift a bit of confectioners' sugar over the top just before serving.
In medium bowl, gradually whisk boiling water into cocoa powder until smooth. Set aside to cool to room temperature, whisking occasionally.
Adjust oven rack so that it is one-third up from bottom of oven. Preheat oven to 325 degrees F. Have ready cookie sheet and an un-greased tube pan.
Into a seperate large bowl, sift cake flour, 1 cup of the sugar (reserve remainder), baking powder, and salt. Add COOLED cocoa mixture, rum, egg yolks, oil, and vanilla. With large, sturdy whisk or hand-held electric mixer, beat briefly just until smooth.
In non-plastic large bowl, combine egg whites and cream of tartar. With clean beaters, beat at high speed until very foamy and white. Gradually add remaining 3/4 cup sugar, about 2 tablespoons at a time. Continue beating until this meringue forms stiff peaks.
With large spatula, gently fold about one-third of the meringue into the cocoa-yolk mixture. Turn this mixture back into the remaining meringue; gently fold together just until no white streaks show in the batter. Turn into tube pan and level batter. Place filled pan on baking sheet, then place into the preheated oven for 45 to 55 minutes.
While cake bakes, ready a cooling rack. When done, the cake should be well-risen (possibly above the edge of the pan), and it will spring back when touched lightly on top with a fingertip.
When done, remove cake from oven with oven mits. Turn the cake gently upside down so it rests on the cooling surface undisturbed. During this time, the cake may shrink slightly and pull away from the pan sides in places--OK.
When cake is completely cooled, gently and carefully turn right side up. Carefully loosen cake from pan sides and tube, using a knife. Turn cake out of pan onto serving plate (it will be served upside down). If bottom of cake sticks to pan, hold serving plate and pan together, and bang GENTLY on table to loosen cake bottom (I have not had this trouble with this particular cake, though). Store airtight at room temperature for up to three days or freeze.
You can sprinkle with powdered sugar sprinkled on top and when you slice, serve with a dolup of whipped cream and some chocolate shavings.