Put the butter and sugar into a mixer bowl and beat with an electric mixer until "creamed"--fluffy and light in color, with no sugar grit easily apparent
Beat in the eggs, one at a time, until they disappear into the creamed butter.
Add one cup of the flour and beat it into the mixture. Add a half-cup of the buttermilk and beat that in. Repeat this step, then one more time, using all of the flour and buttermilk.
Dump the berries and the orange zest on top of the batter. Using a rubber spatula, fold the batter over them. Scraping the sides of the bowl up from the very bottom, keep folding the dough until the berries are well-distributed. This folding technique rather than mixing will keep the berries from breaking.
Coat the insides of two six-pocket muffin tins lightly with butter. (This is not necessary if using non-stick muffin tins. Do not use paper muffin cups.) Put the same amount of muffin batter in each pocket, and bang the tins down on the counter to flatten the tops.
Bake the muffins at 375 degrees, in the center of the oven, for between 20 and 30 minutes--longer for larger muffin tins. If you have a convection oven, use the convect feature. The muffins are ready when the tops are golden brown.
Let the muffins cool for five to ten minutes in the tins. Carefully extract them from the tins and serve immediately.