For yeast dough, put the butter in when you start the second stage. Later, and it won't bind well. Kneading by hand will fix it after a while.
For shortcrust, use cold or soft butter.
For crumble, use very cold butter in thin slices, or it will become shortcrust.
Pay attention when baking. It's not a forgiving craft.
I *do* know all that.
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