Preheat the oven to 400 degrees F. Spray a 12-cup muffin tin generously with baking spray; set aside.
For the popovers: Whisk the milk, flour, eggs, vanilla and 1/2 teaspoon salt in a medium bowl until combined. Drizzle in the melted butter while whisking until combined, then let the batter stand for 5 minutes. Whisk in the baking powder, then divide the batter evenly among the muffin cups (a scant 1/4 cup each). Bake until puffed and browned around the edges, 20 to 25 minutes, rotating the pan halfway through.
For the salad: While the popovers are baking, toss the tomatoes, mozzarella, cucumbers and basil in a medium bowl. Whisk the oil, vinegar, 1/2 teaspoon salt and a few grinds of black pepper in a measuring cup; add to the tomato mixture and toss to coat. Cover the bowl and refrigerate.
To assemble: When the popovers are done, turn them out of the pan, using a small spatula if necessary. Place 2 popovers on each of six serving plates and top each with 1/2 cup of the tomato mixture. Serve immediately.
Never leave a child unattended in the kitchen. Limit the child to tasks that are safe and age-appropriate.
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