Position a rack in the lower third of the oven. Preheat the oven to 450°. Place 1 teaspoon of butter in each cup of a 12 cup standard muffin pan. Transfer the pan to the oven 3-5 minutes before baking to melt the butter and heat the pan.In a medium bowl, vigorously whisk together the milk and eggs until frothy, about 1 minute. Add 1 tablespoon melted butter, vanilla, flour, and salt. Whisk to combine. It's OK if there are small lumps. Carefully remove the pan from the oven and swirl the butter around the cups to grease the pan. Evenly divide the batter between the popover cups, filling them 3/4 of the way full. Transfer to the oven and bake 15-18 minutes, until puffed, golden, and crisp.To make the brown butter. Add 4 tablespoons butter to a skillet set over medium heat. Allow the butter to brown until it smells toasted and is a deep golden color, about 3-4 minutes. Remove from the heat. Combine the sugar, lemon zest, and vanilla powder in a shallow bowl.To make the strawberries. Toss all ingredients together. Let sit 5-10 minutes. Brush each warm popover with browned butter, then roll through the lemon sugar. Smear each popover with butter and top with strawberries. Eat warm!