Baking powder (preferably aluminum free) occasionally appears as an ingredient in a pie crust dough in a very small amount. The actual use of the component varies from baker to baker. In the buttery dough for my rustic fig tart (page 274, and following page) from Baking Style: Art, Craft, Recipes, I included 1/8 teaspoon (in a formula that calls for 2 cups all-purpose flour). The general reason this leavening agent is utilized is to promote a bit of a rise (especially in a rich dough), with the ancillary result of tenderizing the baked crust. In the case of the buttery dough recipe for the free-form tart, the baking powder supports its buttery, sweetened texture.