Preheat your oven to 375°F and lightly butter a 9-inch baking dish. I use a 2-quart round dish, but square works too. The important part is having enough room for the batter to puff up without overflowing.

Whisk together flour, sugar, salt, and vanilla powder in a large bowl. Make sure there aren't any lumps hiding in the flour—gives you a smoother batter. Set this dry mix aside.

In another bowl, whisk together eggs, milk, heavy cream, and melted butter until completely smooth. Pour the wet ingredients into the dry mix and stir until just combined. Don't overmix—a few tiny lumps are totally fine and honestly better than overdoing it.

Scatter strawberries and blueberries across the bottom of your buttered baking dish. Sprinkle the lemon zest right over the berries. Pour your batter carefully over everything—it'll look watery at first, which is exactly what you want.

Bake for 40 minutes until the top is golden brown and puffed up. The center should still jiggle just slightly when you gently shake the pan. I learned the hard way that pulling it out too early leaves it custard-y in the middle, but that's honestly not the worst problem to have.

Let it cool for about 10 minutes before dusting with powdered sugar. The puff will deflate a little—that's normal and doesn't mean anything went wrong. Dust it right before serving so the powdered sugar stays pretty and white.