Step 1In a bowl, sprinkle 2 tablespoons of the gelatin over ½ cup cold water and set aside to give the gelatin time to absorb the water, about 2 minutes. Add ½ cup boiling water and stir until the gelatin is fully dissolved. Stir in ½ cup of the sugar and the root beer until the sugar is dissolved. Skim any foam created by the root beer. Pour into a 2-quart baking dish and refrigerate until firm (3 hours, preferably overnight).
Step 2Once firm, cut the root beer gelatin into 1/2-inch cubes in the baking dish. Using a spatula, carefully loosen the cubes from the dish and mix them around so they are haphazardly arranged in the dish. Refrigerate the root beer cubes while you prepare the vanilla ice cream gelatin.
Step 3In a large bowl, sprinkle the remaining 2 tablespoons gelatin over the remaining ½ cup cold water and set aside for to give the gelatin time to absorb the water, about 2 minutes. Add the remaining 1 cup boiling water and stir until the gelatin is fully dissolved. Add the remaining ¼ cup sugar and stir until the sugar is dissolved.
Step 4In a saucepan, melt the ice cream over high heat until it liquefies. Add the melted ice cream to the gelatin and stir until the mixture is smooth. Cool the mixture to room temperature, then gently spoon over the root beer cubes in the baking dish. Refrigerate until firm. Cut into blocks and serve.