Fill medium pan with an inch of water and bring it to a simmer. In medium heatproof bowl, whisk together eggs and the milk. Put the bowl over simmering water and whisk the egg mixture constantly to warm it up to room temperature. Remove bowl from heat and whisk in the flour, salt, and tarragon just til mixed. The batter should be lumpy. Transfer batter to pitcher with pour spout and let it sit, covered with plastic wrap for about 1 hour at room temperature to tenderize the flour. You can mix the batter a day ahead and keep it, covered in refrigerator. Heat oven to 450. When oven is hot, put a mini popover pan or mini muffin tin in oven and let heat for about 10 minutes. Melt butter in small pan over medium low heat and have a pastry brush ready. Remove hot popover pan from oven and quickly brush the cups well with melted butter. Then quickly pour batter into cups, filling them about halfway. Bake for 15 minutes. This is when they’ll puff up. Then turn oven temperature down to 375 and bake for another 5 to 10 minutes, til popovers have risen up high over the edge of the cups and are nicely browned but not too dark brown. Don’t open oven door at all during first 20 minute of baking; the popovers are being leavened by steam and could easily deflate. Check after 20 minutes to make sure they aren’t getting too dark. Serve at once. Makes 12 to 24 mini popovers, depending on pan size.