1. To make the cookies: In the bowl of a stand mixer or with a handheld mixer, cream butter and sugars until creamy, about 3 minutes.
2. Add in the eggs one at a time, beating well between each addition. Add in the vanilla extract.
3. Add in baking powder, baking soda, salt, pumpkin pie spice and 1 cup of flour. Mix well.
4. Add in remaining 2 cups flour, mix until no streaks of flour are present.
5. Add in the oatmeal and raisins; mix until thoroughly combined. Cover with plastic wrap and refrigerate 1 hour.
6. Preheat oven to 350ºF. Line 2 sheet pans with parchment paper or silicone mats.
7. Using a 1/4 cup scoop or 2 spoons, drop mounds of dough, spacing 2 inches apart onto prepared pans.
8. Bake for 12-15 minutes, or until the edges are golden brown and the middle is slightly under-baked. Remove from oven, let the cookies cool on the pan for 1 minute, then transfer to a wire cooling rack. Cool completely.
9. Make the buttercream frosting- this can be made 2 days ahead. It can be stored on the counter or in the refrigerator. If refrigerated bring up to room temperature before using.
10. Place the egg whites and sugar in a metal bowl of a stand mixer. Place the bowl over a pan of simmering water (make sure the bowl does not touching the water). Whisk the egg white mixture constantly until sugar is dissolved and mixture comes to approximately 160ºF.
11. Immediately place the bowl on the stand mixer with the whisk attachment, whisk the egg mixture on high until the frosting has become thick and glossy, forming stiff peaks. Continue to whip frosting until the bowl feels room temperature to the touch, about 10 minutes.
12. Switch the whisk attachment to the paddle attachment. With the mixer on low, add the butter, 1 tablespoon at a time, beating until smooth after each addition. If frosting appears curdled or clumpy at any time, increase speed to medium-high and beat until smooth.
13. Add in the vanilla extract and the pinch of salt. If needed, continue beating until smooth.
14. To assemble: add 1/4 cup of frosting in between 2 cookies. Store in an airtight container for 3 days or freeze for 2 months. If frozen, bring to room temperature before serving.