Petit-déjeuner could get more petit. Wholesale dairy prices have nearly doubled since April 2016, and recent studies showing that butter is healthier than previously believed have stoked Western demand. Now the French Bakers’ Federation warns that the price of croissants, which are about 25 percent butter, could skyrocket in the coming months. Even worse, there might be a shortage of the iconic flaky pastries. That could bring pain — and not the crusty kind — to family bakeries that can’t eat the extra cost or risk losing clientèle with higher prices.