Preheat the oven to 325 degrees F (165 degrees C).

Combine flour, butter, confectioners' sugar, poppy seeds, vanilla extract, and salt in the bowl of a food processor; pulse to combine. Dough will be dry and crumbly.

Press dough into a 11x7-inch baking pan. Pierce entire surface with a fork to prevent the dough from rising during baking.

Bake in the preheated oven until edges start to turn slightly brown, 30 to 40 minutes.

Remove shortbread from pan and cut into 12 equal-sized bars while still warm. Place on a wire rack to cool.