Heat your oven to 375°F and grease a 9-inch round baking dish or cast-iron skillet with butter. Make sure you get into the corners and coat the bottom really well — this prevents sticking. Don't skip this step even though it seems obvious (I learned that the hard way).

In a medium bowl, whisk together the flour, granulated sugar, and salt. You want it super combined so there aren't any lumps hiding anywhere. This takes about 30 seconds of actual whisking.

Add the eggs to your flour mixture and beat them in with a fork or whisk until mostly combined. Pour in the milk slowly while stirring, then add the melted butter and vanilla powder. Stir until you've got a smooth batter with no lumps — it'll look like thin pancake batter.

Toss your sliced strawberries and blueberries together in a separate bowl with the lemon zest. This gets everything coated and ready to distribute evenly. You'll want them somewhat mixed so you get both berries in every slice.

Pour about half your batter into the prepared baking dish. Scatter your berry mixture over the batter layer — don't stress about even distribution because they'll settle and shift anyway. Trust me on this.

Pour the remaining batter over the berries, being gentle so you don't crush them. The batter won't completely cover everything and that's totally fine. It'll rise up around the berries and create this beautiful custard texture that's kinda puffy.

Bake for 35-40 minutes until the top is golden brown and the center jiggles just a tiny bit when you shake the pan. A toothpick inserted near the center should come out mostly clean. Let it cool for 5 minutes, dust with powdered sugar, and serve warm.