In a large bowl, beat the heavy whipping cream with an electric mixer until stiff peaks form (about 3-5 minutes).
Combine the Ingredients:

Gently fold the mashed bananas and vanilla extract into the whipped cream. Be careful not to deflate the whipped cream.
Fold in the peanut butter mixture until everything is smooth and well combined. Optionally, add powdered sugar to taste for extra sweetness.
Freeze the Mixture:

Line a loaf pan (or any freezer-safe container) with plastic wrap or parchment paper. Pour the semifreddo mixture into the pan and smooth the top. Top with chopped peanuts, if desired.
Freeze:

Cover the pan with plastic wrap and freeze for at least 4-6 hours, or until firm.