Make the bombe: Line a 7-cup bombe mold or deep bowl with plastic wrap. Spread raspberry sorbet into mold, pressing evenly into bottom to prevent air pockets. Freeze 1 hour. Spread blueberry sorbet evenly over raspberry. Freeze 1 hour. Spread vanilla ice cream over blueberry. The ice cream should come to top of mold. Place dacquoise round on ice cream, pressing gently to adhere. Wrap bombe in plastic wrap, and freeze overnight.
Dip bottom and sides of mold in hot water, unwrap top of mold, then invert mold onto a serving platter (repeat dipping if necessary). Remove plastic wrap; wipe off excess melted ice cream and sorbet. Refreeze at least 1 hour and up to overnight.
Make the meringue frosting: Bring sugar and water to a boil in a small saucepan, stirring until sugar dissolves. Cook, undisturbed, until a candy thermometer registers 248 degrees.
Meanwhile, whisk egg whites with a mixer on low speed until foamy. Add salt and cream of tartar. Raise speed to medium, and whisk until soft peaks form, about 8 minutes. Reduce speed to low, and add hot syrup in a slow, steady stream down side of bowl. Raise speed to high, and beat until thick, fluffy, and cool, about 7 minutes.
Fit a large pastry bag with a large star tip (such as Ateco #827). Pipe meringue over cake to completely cover with rosettes, pulling up as you pipe to form points. Freeze at least 30 minutes and up to 6 hours.
To serve, lightly brown meringue all over with a small kitchen torch. Serve immediately with coulis, blueberries, and dacquoise cookies.
To finish Martha's baked Alaska bombe, meringue was piped over layers of ice cream and sorbet. The creation was then briefly frozen and lightly browned with a kitchen torch before being served with fresh fruit and dacquoise cookies.