1. Preheat oven to 425°F.
2. Spray heavy baking sheet with nonstick spray.
3. Whisk both flours, sugar, and baking soda in large bowl to blend.
4. Add butter and cut in until it is reduced to pea-size pieces.
5. Add buttermilk; stir until shaggy dough forms.
6. Turn dough out onto lightly floured work surface.
7. Knead until dough comes together, about 10 turns.
8. Shape dough into 7-inch round.
9. Place dough on prepared baking sheet. Cut a large X, 1/2 inch deep, in top of dough.
10. Bake bread until deep brown and bottom sounds hollow when firmly tapped, about 40 minutes.
11. Transfer bread to rack and cool completely.

Read more about Irish Soda Bread here.