Preheat your oven to 400° F / Butter the bottom of your pie dish generously.
In the bowl of your food processor add the flour, baking powder and salt and pulse a few times to combine. Alternatively, if making it by hand, use a big bowl to mix the dry ingredients together.
Add the cubed, chilled butter and pulse until the mixture resembles coarse sand. If mixing by hand, use a pastry cutter or two forks to break apart the butter into small little pieces, until your mixture resembles coarse sand. Avoid using your fingers, as this will melt the butter.
Add the sour cream and pulse (or mix) until the dough comes together and holds it's shape when pressed together.
Transfer the dough onto a floured surface and knead it gently until it is nice and smooth, adding a teaspoon of ice water at a time, if needed. Avoid adding too much water, you do not want your dough to be sticky.
Shape the dough into a disk and wrap it in parchment paper or cling wrap and allow it to rest in the fridge for 30 minutes.
After your dough has rested, flour your work surface and rolling pin and roll out the dough into a large rectangle, about 1/2" thick, sprinkling flour as needed and making sure to turn the dough a few time to prevent it from sticking.
Fold the dough in half, and then in half again, into a square and roll it out once more, sprinkling with flour as needed to prevent it from sticking, into a circle large enough to cover your baking dish.
Roll the dough onto your rolling pin and carefully transfer it to your baking dish by unrolling it onto the dish. Fold any overhang inwards, tucking in the edges as you crimp them with your thumb and index finger all the way around.