From America's Test Kitchen Season 9: Breadmaking, Simplified
Authentic Irish soda bread has a tender, dense crumb and a rough-textured, thick crust—definitely a departure from the more common Americanized soda bread, which is closer to a supersized scone. We wanted to try our hand at the authentic version of this bread, which relies on a simple ingredient list of flour, baking soda, salt, and buttermilk.
Our first tests focused on flour. A loaf made with all-purpose flour produced a doughy, heavy bread with an overly thick crust. To soften the crumb, we added some cake flour to the mix, and this made a difference. (It also made historical sense. Because of Ireland’s climate, the wheat grown there is a soft, low-protein variety more similar to cake flour than to American all-purpose, which is relatively high in protein.) A version made with all cake flour, however, was heavy and compact. A ratio of 3 parts all-purpose flour to 1 part cake flour proved best. With only the four basic ingredients of flour, buttermilk, baking soda, and salt, our bread was lacking in flavor and still a little tough; we turned to sugar and butter. Traditionally, very small amounts of butter and sugar are sometimes added to Irish soda bread, so we felt justified in using a minuscule amount of each. The sugar added flavor without making the bread sweet, and the butter softened the dough just enough without making it overly rich.
Makes 1 loaf
If you do not have a cast iron pan the bread can be baked on a baking sheet although the crust won’t be quite as crunchy. Soda bread is best eaten on the day it is baked but does keep well covered and stored at room temperature for a couple of days after which time it will become dry.