Preheat oven to 400°F. Line a baking sheet with parchment.
Whisk mascarpone, powdered sugar, and vanilla until smooth.
Toss berries with lemon juice.
Brush a phyllo sheet lightly with butter; top with a second sheet. Keep unused sheets covered.
Spread one quarter of the mascarpone mixture in a thin line near one edge and top with one quarter of the berries.
Roll tightly; place seam-side down on prepared sheet. Repeat with remaining phyllo, filling, and berries to make 4 rolls.
Brush all 4 rolls with the remaining butter and sprinkle with the coarse sugar.
Bake 10–12 minutes until golden and crisp. Cool 5 minutes, then slice.