Preheat oven to 450º.
In a large bowl, stir together the flour, baking powder, baking soda, salt and sugar.
Add cubed butter to the bowl and cut into the dough with a pastry blender until the butter is pea sized. Do quickly so butter doesn't warm up.
Stir in buttermilk, do not over work the dough. It should be sticky and loose, barely holding together.
Turn dough onto a floured surface and knead a few times to bring together.
Press dough gently into a 1 inch thickness. Using a biscuit cutter, cut biscuits. Try not to twist cutter at all, it seals the edges and prevents the biscuits from rushing into flaky layers.
Arrange biscuits on a baking sheet, lightly touching each other. This helps them rise in the oven. Brush tops with buttermilk.
Bake until golden brown, 10-15 minutes.