To make the dough in a food processor: In a food processor, combine the flour, sugar, salt, and baking powder, and process to mix. Add the butter and process until the mixture resembles a coarse meal. Put the work bowl in the refrigerator to chill for 15 minutes.
Add the sour cream to the flour mixture and process until the dough comes together in the bowl. Gather the dough into a ball and knead it several times on a lightly floured surface. Form it into a 1-inch-thick disk, wrap it in plastic, and chill for at least 30 minutes before rolling.
To make the dough by hand: In a medium bowl, combine the flour, sugar, salt, and baking powder. Add the butter and cut it into the flour with a pastry cutter or two knives until it resembles a very coarse meal. Alternatively, using a pinching motion, mix the butter into the flour with your fingers, then rub the butter and flour between the palms of both hands to blend it until the mixture is the texture of coarse meal. Chill the dough in the refrigerator for 15 minutes. Add the sour cream and blend it in with the pastry cutter or a fork. Knead and squeeze the dough 7 or 8 times to incorporate any loose bits. Gather the dough together into a rough ball (it will be a coarse mass), flatten it into a 1-inch-thick disk, and wrap in plastic wrap. Refrigerate at least 30 minutes before rolling.
You can roll and cut the dough into shapes up to one week ahead of baking, arrange on a baking sheet, wrap well, and freeze. There is no need to defrost them before baking.
To roll out the dough: Let the dough sit at room temperature for about 15 minutes before rolling. Sprinkle the work surface lightly and evenly with a little flour. Rub the rolling pin with flour as well. Place the dough in the middle of the work surface. Beginning at one edge, press the rolling pin down onto the dough to flatten it, moving across the dough in increments. Then, moving from the center of the dough outward, begin to roll the dough, adding more flour as necessary to keep the dough from sticking. Roll the dough gradually in all directions, flattening as you go, to form a large circle about 14 inches in diameter; do not roll it thinner than 1/8 inch. If the dough cracks or pulls apart, moisten the torn edges with a little water (using your finger or a brush) and press together to seal. Dust lightly with flour if the surface of the dough is sticky.
The dough is ready to use in a tart or to cut out. To transfer the dough to a 9-by-10-inch tart tin or baking sheet, place the rolling pin gently on one edge of the dough and roll the dough up over the pin - then you can move it wherever your want.
Sweet Pastry Variation: This variation produces a pastry that is sweeter and crisp like a sugar cookie. Increase the sugar to 3 tablespoons and add 1/2 teaspoon pure vanilla extract or grated lemon zest.