In a medium saucepan over low heat, combine sweetened condensed milk, semi-sweet chocolate chips, butter, and vanilla extract. Stir until melted and smooth (about 5-7 minutes).

Pour the melted fudge into a lined 8x8-inch dish, spreading evenly. Let cool slightly.

In a blender, combine fresh strawberries, powdered sugar, and lemon juice. Blend until smooth.

Spoon dollops of the strawberry mixture on top of the fudge, and gently swirl into the fudge for a marbled effect.

Refrigerate for at least 2 hours or until set.

Cut into squares and garnish with chocolate shavings or fresh mint leaves if desired.