Chop the rosemary well so that there are no large, sharp pieces. Add the rosemary, flour, baking powder, salt, and some freshly cracked pepper to a food processor and pulse until the mixture is evenly mixed (or just stir them together in a bowl).
Cut the butter into pieces and add it to the food processor. Pulse the mixture until the butter is completely incorporated and no chunks remain. Or, cut the butter into the flour mixture with pastry cutter, two knives, or just with your hands until the butter is completely worked into the flour mixture.
Slowly add cold water to the food processor while pulsing, just until it forms a dough (or stir it in by hand until a dough forms). Depending on the humidity and moisture level in your flour, it will take between 1/2 and 3/4 cup water. I used approximately 2/3 cup.
Preheat the oven to 400 degrees. Turn the dough out onto a lightly floured surface and roll it out until it is approximately 1/16th inch thick. Use a pizza cutter or knife to cut the dough into small rectangles, squares, or triangles. Prick each “cracker” with a fork. Carefully transfer the cut crackers to a baking sheet lined with parchment paper.
Sprinkle the crackers lightly with salt, if desired. Bake the crackers for 20-25 minutes, or until they are golden brown. The total amount of baking time needed will depend greatly on the thickness and size of the crackers, so watch them closely. Allow the crackers to cool and then store in an air-tight container at room temperature.