Preheat oven to 400°. Whisk together cornmeal mix, flour, sugar, lemon zest, and thyme in a large bowl. Add buttermilk, melted butter, and eggs. Whisk together just until blended. Spoon batter into lightly greased shiny madeleine pans, filling three-fourths full. Bake, in batches, 16 to 18 minutes or until golden brown. Remove from pans immediately. Serve hot, or cool completely on wire racks (about 20 minutes), and freeze in zip-top plastic freezer bags up to 1 month. To serve, arrange desired amount of madeleines on a baking sheet, and bake at 350° for 5 to 6 minutes or until thoroughly heated.
The traditional shiny heavy-gauge, tinned steel madeleine pan, which makes one dozen 3- x 2-inch madeleines, yields the prettiest results. (Dark nonstick versions tend to over-brown.) Look for shiny pans in bake shops, or order online from bakeandcookco.com ($17.95) or amazon.com. To prevent over-browning if you do use a dark nonstick madeleine pan, wrap the bottom of the pan with a sheet of heavy-duty aluminum foil, shiny side out.