Preheat the oven to 375ºF (190Cº). Line an 11-inch by 17-inch (28 cm by 43 cm) baking sheet with a piece of parchment paper.
Combine the flour, salt, sugar, and baking powder in the bowl of a food processor. Pulse a few times to blend. Add the butter and pulse about 10 times, until the flour resembles a coarse meal. Add 1/3 cup cold water, and pulse until it forms a scraggly-looking dough.
Lightly flour a counter or cutting board. Scrape the dough onto the counter, and knead a few times to form a smooth dough. Cover with a clean kitchen towel, and let rest for 15 minutes.
On the same counter or cutting board, roll dough out into a 1/8-inch thick sheet. Cut dough into 1 ½-inch squares. You can re-roll the scraps once. Place the cutouts on the prepared pan. Use a skewer to prick little marks in the top, like you see on crackers from the grocery store, if desired. Sprinkle lightly with some salt flakes.
Bake until the crackers are barely golden around the edges, 14 to 15 minutes. Remove from oven and let cool completely. Store crackers in an airtight tin for up to 1 week.