Whisk the flour, baking powder, and baking soda in a large mixing bowl if making by hand, or into the large bowl of a stand mixer fitted with the paddle attachment. Add the sugar and salt and stir to mix. Cut the butter into ½-inch cubes and scatter the cubes over the dry ingredients. If you are using a mixer, pulse on and off so that you don’t break down the butter too much-you want to end up with a coarse mixture with pea-sized lumps of butter visible. If you are mixing by hand, combine the mixture using your hands until the mixture is thoughroughly combined with large pea-sized lumps of butter.

Add the buttermilk all at once along with the lemon zest. Mix gently with a rubber spatula by hand or on low speed if using the mixer. Add the rhubarb and continue to mix just until you have a dough that holds together. If the mixture seems dry, add a little more buttermilk. You still want to see some of the butter pieces at this point.

Dust your work surface with flour, and turn the dough out onto it. Using your hands, pat the dough into a rectangle about 9 inches long, 5 inches wide, and 1½ inches thick. Brush the top with the melted butter and sprinkle with the sugar. Using a chef’s knife, cut the dough into triangles of desired size. Transfer the triangles to the prepared baking sheet. 

If you are freezing some (or all of them)-place the scones on a tray lined with parchment paper and "flash freeze" for about 20 minutes.  Transfer to a large ziploc bag or other freezer safe storage container. 

Bake the scones until the tops are lightly browned, 25 to 30 minutes. Remove from the oven and serve immediately.