In a large bowl, using a fork, stir 5 cups flour with baking powder and salt. In another large bowl, using an electric mixer, beat butter with sugar on medium-high for 2 min. Beat 1 tbsp dark sesame oil into butter mixture. Beat in eggs, 1 at a time, scraping down side of bowl as necessary. Then beat in vanilla. Using a wooden spoon, gradually stir in flour mixture just until evenly blended. (Over-mixing toughens cookies.)
If dough is too sticky to form into a ball, stir in remaining 1/2 cup flour, 2 tbsp at a time, until dough is no longer sticky and easy to roll. (You may not need all the flour.) Divide dough into 4 portions. Shape each into a ball, then flatten slightly into a disc. Wrap separately with plastic wrap and refrigerate until firm, at least 1 hour or up to 1 week. Or overwrap with heavy foil and freeze up to 1 month.
When ready to bake, preheat oven to 375F. Lightly spray or oil 2 baking sheets or line with parchment paper. On a lightly floured surface, roll dough no thicker than 1/4 in. Cut out shapes with a cookie cutter. Place on baking sheets, about 2 in. apart. Sprinkle with 1/4 cup each black and plain sesame seeds. Bake in centre of preheated oven until cookies are pale golden, 8 to 10 min, depending on thickness. Place cookie sheet on a rack for 5 min. Then remove cookies to a wire rack to cool completely. Repeat with remaining dough. Store in an airtight container in the refrigerator up to 1 week or freeze up to 1 month.