In a large mixing bowl whisk together the flour, salt, baking powder, and spices. Add the brown sugar.
Stir in the egg, molasses, and vegetable oil. (I used my stand mixer on low.)
Mix this thoroughly, then wrap in plastic and chill for at least 3 hours or overnight.
Preheat oven to 300°F.
Prepare your flat cookie sheet by wrapping with foil; then spread a thin layer of oil on the foil.
Remove the dough from the refrigerator, it will still be fairly soft.
Lay the dough in the middle of your baking sheet and sprinkle lightly with flour. (If you put the baking sheet on top of a dish towel, it should help to keep it stable while rolling out the dough.)
Starting from the middle, roll the dough to each edge of the baking sheet. Add light amounts of flour as necessary to keep from sticking to the rolling pin. Try to get the dough as evenly thick as possible and as close to straight edges as possible.
Bake for 35 - 40 minutes. Watch it carefully, because ovens do differ and you don't want the edges to burn, but you do want the gingerbread to be totally baked.
Place the pattern on the hot bread and immediately cut the pieces out, then place on a rack to cool. The pieces should be very hard. If they do not cool hard, then you can put them back in the oven for a few minutes to harden.