In a medium saucepan, combine fresh cranberries, 1/2 cup sugar, 1/4 cup water, and the juice of half a lemon. Cook over medium heat for about 10 minutes, stirring occasionally, until the cranberries burst and the mixture thickens slightly.

In a small bowl, sprinkle 2 tablespoons of gelatin over 1/4 cup of cold water. Let the mixture sit for about 5 minutes to allow the gelatin to soften.

Gently warm the cooled cranberry mixture over low heat, stirring just until slightly warm. Stir in the softened gelatin until completely dissolved.

In a large mixing bowl, pour in 1 cup of chilled heavy whipping cream, followed by the remaining 1/2 cup sugar and 1 teaspoon of vanilla extract. Whip on medium-high speed until stiff peaks form, approximately 3-5 minutes.

Once the whipped cream is ready, gently fold in the cooled cranberry-gelatin mixture.

Carefully pour the cranberry mousse mixture into the prepared graham cracker crust, smoothing the top with a spatula.

Cover the pie and refrigerate for at least 4 hours, preferably overnight.

Just before serving, top the chilled pie with additional whipped cream and sprinkle with cranberries dusted with powdered sugar.