1. Preheat oven to 425ºF. Line a baking sheet with parchment paper. In a bowl, combine raspberries, cornstarch, lemon zest and 4 Tbsp. sugar.
2. Lay 1 phyllo sheet on a work surface, short side facing you, keeping remaining sheets covered in plastic wrap. Brush sheet lightly with butter and lay a second sheet on top. Repeat with remaining butter and phyllo. (You will have some butter left over.)
3. Spoon raspberry filling in a 2-inch-thick line along long edge of dough, leaving 2 inches at top and bottom. Sprinkle with white chocolate chips. Roll up tightly, fold ends under, and place seam side down on baking sheet. Brush with a bit of remaining butter. Sprinkle with remaining 1/2 Tbsp. sugar.
4. Bake until golden brown, 15 to 20 minutes. Let stand on baking sheet on a wire rack for 15 minutes, then slice and serve warm, or let cool and serve at room temperature.