Preheat oven to 375°F; butter a 9x13-inch dish.
Gently toss raspberries, 1/2 cup sugar, lemon juice, 2 teaspoons zest, and cornstarch in a large bowl. Stand 15 minutes; pour into dish.
Whisk flour, remaining 1/4 cup sugar, baking powder, and salt in another bowl. Cut in cold butter with pastry cutter or fork to coarse crumbs.
Stir milk and vanilla into flour mixture just until combined. Drop batter spoonfuls over filling, leaving steam gaps.
Sprinkle with remaining teaspoon zest; bake 38-42 minutes until topping is golden brown and filling bubbles.
Cool 15 minutes; serve warm with vanilla ice cream or whipped cream.