Sweet bread is based on Portuguese massa sovada. This bread has been around a long time. It predates pasteurized milk. So people continue to scald the milk in sweet bread recipes—in a lot of recipes.
To incorporate melted butter, or in this case a solid fat, you just simply have to melt it.
Regarding the windowpane test, The test is simply to check for gluten development.
A lean dough has no fat. this is your basic bread dough, flour, yeast, salt, water. it’s a lot easier to develop gluten in a lean dough since no ingredients interfere with the gluten development.
An enriched dough contains fat, sugar, and dairy in addition to the flour, yeast, and salt. The fat, sugar, and dairy inhibit gluten development. Since these ingredients interfere with gluten development you have to knead for a longer time.
Since it takes longer to develop gluten, enriched doughs often include instructions for a windowpane test. It’s not some thing exclusive to any particular type of dough. It’s simply to avoid under kneading an enriched dough.
Most people don’t perform the window pain test correctly. They take two small a piece of dough, tear it, think they don’t have enough gluten and end up developing too much gluten in the dough.
Knowing how to test if your dough is ready to bake is also very important. You can’t just simply leave it to rise for two hours and assume it’s
Using a proper container so you can actually see window has doubled in size
Test dough to make sure it is properly proofed
Windowpane test video