The best way to bake croissants is to place them in a hot oven so the layers open up and the butter between the layers is cooked off instantly leaving no time for it to melt. But, you also want them to continue cooking without becoming too dark on the outside. The ideal temperature for homemade croissants I find is 200C / 392 F for 10 minutes, then reduce oven temperature to 190 C/ 375F for 15 minutes. Tent the croissants if they are becoming brown quickly.