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 there are no streaks of flour are present.
5. Add in the oatmeal and mix until oatmeal and raisins are thoroughly combined.
6. Refrigerate 1 hour.
7. Preheat oven to 350ºF
8. Using a 1/4 cup scoop or 2 spoons, drop mounds of dough, space 2 inches apart onto sheet pans lined with parchment paper or silicone mats.
9. Bake for 12-15 minutes. Remove from oven, let the cookies sit on the pan for 1 minute, then transfer to a wire cooling rack. Cool completely.
10. 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 serving.
11. Place the egg whites and sugar in a metal bowl of a stand mixer. Place bowl over a pan of simmering water, not touching the water. Whisk or stir constantly until sugar is dissolved and mixture comes to approximately 160ºF.
12. Immediately place the bowl on the stand mixer and whip on high using the whisk attachment until frosting has become thick and glossy, forming a stiff peak. Continue to whip frosting until the bowl feels room temperature to the touch, about 10 minutes.
13. Switch the whisk attachment to the paddle attachment. With the mixer on low, drop in 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.
14. Add in the vanilla extract and the pinch of salt. If needed, continue beating until smooth.
15. To assemble cookies, add about 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.
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