Combine raspberries, lemon zest, and sugar in a medium saucepan over medium heat. Cook for 5-10 minutes, stirring occasionally, until the raspberries begin to break down and release their juices. In a small bowl, whisk together cornstarch and water to create a smooth slurry, then stir this mixture into the raspberries. Continue cooking for 1 minute, stirring constantly, until the filling thickens noticeably—you should see it coat the back of a spoon. Remove from heat, stir in vanilla extract and a pinch of salt, then transfer to a bowl to cool completely. I like to let the filling cool to room temperature before assembly because warm filling can make the phyllo soggy and difficult to work with.
Preheat your oven to 400°F and line a baking sheet with parchment paper or lightly grease it. While the oven heats, remove your phyllo sheets from the package and lay them on a clean, dry work surface, covering them loosely with a damp kitchen towel to prevent them from drying out. Have your melted butter and breadcrumbs ready and within arm's reach, as you'll be working quickly once you begin layering the phyllo.
Place the first phyllo sheet on your prepared baking sheet and brush it lightly with melted butter using a pastry brush. Lay the second phyllo sheet directly on top and brush with butter again. Repeat this process with sheets 3 and 4, building a base of 4 buttered phyllo layers. This creates a sturdy foundation that will hold the filling without tearing. I find that using 4 layers for the base gives you enough structural support without making the strudel too thick and doughy.
Sprinkle the breadcrumbs evenly down the center of your phyllo base, leaving about 2 inches of space on each long side. This creates a barrier that prevents the filling from making the phyllo soggy. Spoon the cooled raspberry filling from Step 1 down the center on top of the breadcrumbs. Now layer the remaining 4 phyllo sheets on top of the filling, brushing each sheet with melted butter as you go, just as you did with the base layers.
Fold the long edges of the phyllo over the filling, pressing gently to seal them together and create a neat packet. Now comes the tricky part—carefully slide a large spatula or baking sheet under the strudel and flip it seam-side down onto the baking sheet so the seam is now underneath and hidden. Brush the entire top surface generously with any remaining melted butter, which will help the phyllo turn golden and crispy as it bakes.
Place the strudel in the preheated 400°F oven and bake for 15-20 minutes, until the phyllo is deep golden brown and crispy. The exact time depends on your oven, so start checking around 15 minutes. Remove from the oven and let it cool on the baking sheet for about 10 minutes—this resting time allows the filling to set slightly so it won't ooze out when you slice it.
Once cooled slightly, transfer the strudel to a cutting board or serving platter. Dust generously with powdered sugar, which adds a beautiful contrast to the golden phyllo and a touch of sweetness. Slice with a sharp, serrated knife using a gentle sawing motion to avoid shattering the crispy phyllo layers. Serve while still warm or at room temperature.
