Combine the sugar and water in a heavy saucepan and bring to a boil without stirring. Cover and let boil until the sugar is completely dissolved, 3 to 5 minutes. Uncover and let the mixture boil until it reaches the soft ball stage, 240°F. on a candy thermometer.

While the syrup is cooking, beat the egg whites with the salt and cream of tartar until very stiff peaks form.

When the syrup reaches 240°F., pour 1/4 cup of the syrup into the stiffly beaten egg whites, drop by drop, beating at high speed. Gradually add the rest of the syrup in a fine stream and beat for 8 to 12 minutes, or until the mixture is cool. There must be no heat left in the meringue or the butter will melt.

In a separate bowl, cream the butter until light and fluffy. Add the butter to the cooled meringue mixture, 3 tablespoons at a time, beating well after each addition. The frosting should gradually become very thick and creamy. If it does not thicken properly, the meringue may have been too warm; stir over a bowl of ice until the frosting thickens. Beat in the vanilla extract and the Grand Marnier.

The buttercream should be used immediately. Keep the buttercream cool over a bowl of ice water, stirring often, but do not refrigerate it. Frost each layer with an undercoat, and refrigerate immediately.