Whisk together the flours, sugar, baking soda, cream of tartar and salt in a bowl.
Cut butter into the dry ingredients until it is coarse crumbs.
Add in buttermilk with a fork until the dough begins to come together.
Turn dough onto floured service and knead just until the dough becomes cohesive and bumpy.
Do not knead until smooth.
Form the dough into a 6 inch round loaf - about 2 inches thick.
Place on a baking sheet covered in parchment paper.
Cut a large 1/2 inch deep X into top of loaf with a knife.
Bake until golden brown and a toothpick comes out clean - about 40 minutes.
Cool and then brush loaf with melted butter.
Notes
To avoid dry and crumbly soda bread, use a combination of all purpose flour and cake flour.
Soda Bread is often made with raisins or nuts but I am kind of a plain jane some time so I left mine plain or I prefer to say "classic." If desired, add in 1 cup of raisins or 1/2 cup raisins and 1/2 cup of nuts.
Don't overwork the dough. It should be a rough, shaggy dough.
Be sure and make a cut into the top of the dough before baking as this helps the bread cook all the way through.