Preheat oven to 450 degrees.  Grease a baking sheet or line a sheet with parchment paper.

In a medium or large food processor fitted with the blade attachment, add the flour, sugar, salt, baking powder, and baking soda. Pulse a few times, until well mixed.

Add the butter pieces and pulse until fine crumbs form. Add the lemon zest and poppy seeds and just pulse briefly to combine.

See notes on an alternate method, but what I usually do is then turn the food processor on low (or pulse) and trickle the buttermilk in while it's running, until the dough comes together. Try and mix as little as possible...it'll be a little sticky/wet and that's fine.

Lightly coat your workspace with flour and turn the dough out onto it.  You may need to sprinkle a little more flour on it if it's super wet.  Gently knead a couple times and pat until it's about 1/2-inch thick, then rotate it 90 degrees, fold it in half, and gently roll or pat it out again until it's 1/2-inch thick. 

Do that again one or two more times, sprinkling with a little flour in-between if it's too sticky (the folding gives you flakier layers).  Finally pat or roll the dough to 3/4-inch thickness (it should no longer be sticky).  I prefer a round-ish shape.

Using a round biscuit cutter, press or punch straight down to cut the dough and transfer the biscuits to your prepared pan (spaced about an inch apart).  You can cut them really close to each other, and then with the scraps smoosh them together and cut your last couple.

You can bake as-is, or I like to just get a little milk or cream on my finger and smear it on top before baking.  You can also refrigerate for 15 minutes before baking to really solidify the butter again (I tend to be too lazy).

Bake until the tops are golden brown and crisp, about 15-18 minutes. Let cool for 5 minutes on the pan before serving hot.