The recipe says to mix in the butter... Is the butter melted or are you mixing in softened butter?

I have a French baking book from Canada whose recipes also seem to be written this way, but I usually use softened butter that I beat with sugar in the usual way to aerate the mixture, then add the eggs one at a time and the additional liquids. I would sift the dry ingredients and whisk in the nuts, then add to the batter in separate increments. Perhaps others will weigh in!