In a medium saucepan, melt dark chocolate and sweetened condensed milk over low heat, stirring continuously until smooth and glossy, about 5 minutes. Add vanilla extract and stir to combine.

Pour the fudge mixture into a lined square pan, spreading it evenly. Let it cool at room temperature for about 30 minutes before adding the strawberry swirl.

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

Once the fudge base has cooled, drop spoonfuls of the strawberry mixture on top and use a knife to create swirls without mixing completely.

Place the pan in the refrigerator for at least 2 hours, allowing the fudge to set firmly. Cut into squares after setting.