In a blender or food processor, combine the frozen berries, Greek yogurt, honey or maple syrup, and vanilla extract. Blend until smooth and creamy.
Taste the mixture and adjust sweetness if needed by adding more honey or maple syrup.
Transfer the berry yogurt mixture into a freezer-safe container. Smooth the top with a spatula.
Cover the container with a lid or plastic wrap and freeze for at least 2 hours, or until firm.
Before serving, let the frozen yogurt sit at room temperature for about 5-10 minutes to soften slightly for easier scooping.
Scoop the frozen berry yogurt into bowls and top with granola and fresh mint leaves if desired.