In a small, microwaveable bowl, sprinkle the gelatin over 3 Tbs. water; let soften for at least 5 minutes. Microwave on high to melt the gelatin, 15 to 30 seconds.
In a heavy-duty 2-quart saucepan over medium-high heat, combine 1 cup of the sugar and 6 Tbs. water and stir constantly until the syrup is bubbling, 2 to 3 minutes. Remove the pan from the heat.
Beat the egg whites until foamy. Add the cream of tartar and beat until soft peaks form. Gradually beat in the remaining 3 Tbs. sugar until stiff peaks form.
Have ready a 2-cup heatproof measuring cup. Return the pan of syrup to medium-high heat and boil until a candy thermometer registers 248°F (firm ball stage). Pour the syrup into the cup to stop the cooking. Immediately pour a small amount of syrup over the whites with the mixer off. Immediately beat at high speed for 5 seconds. Stop the mixer and add a larger amount of syrup. Beat at high speed for 5 seconds. Continue with the remaining syrup. Lower the speed to medium, add the gelatin mixture, and beat on medium speed for 2 minutes. Decrease the speed to low and continue beating until the bottom of the bowl is no longer warm to the touch, about 10 minutes.
Fold one-third of the meringue into the lemon cream. Repeat twice more until all of the meringue is folded into the lemon cream.
Split the cake into 4 layers.
Place largest cake layer on a cake plate. Top with 1/4 of the cream mixture. Repeat layers ending with cream mixture. As filling squishes out of the sides use it to lightly frost the layers.
Insert 3 wooden skewers into cake to keep it from sliding. They'll also be taller than you cake and keep your plastic wrap off the top of the cake when storing. Trim them below the top of the cake with scissors and hide with the cream when ready to serve.
Cover and chill at least 12 hours. I prefer a cake dome, but you can use plastic wrap.