Preheat oven to 400°F. Line a baking sheet with parchment paper.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda and salt.
Cut in the butter until it resembles coarse crumbs. Stir in the poppy seeds.
In a small bowl, whisk together the egg, buttermilk or sour cream (or yogurt). Add to the dry ingredients and mix until just combined.
Turn out onto a lightly floured surface and knead gently 4-5 times. Pat into an 8-inch circle about 1 inch thick. Cut into 8 wedges and place on prepared baking sheet.
Bake in preheated oven for 12-15 minutes or until golden brown. Serve warm.