Poppy seed Cake

Recipe originally from here, found in an issue of Farm Journal
This cake is delicious—moist, not too rich, with a delicate flavor. I baked it in a 13x9” pyrex pan, but it could also be baked in regular cake pans, I think.
I have made it with a traditional buttercream frosting, but the frosting flavor somehow covered up the flavor of the cake itself, we thought. The sour cream-cream cheese frosting below is better with this cake, but it really could just go with a dusting of powdered sugar or a simple powdered sugar glaze.

Ingredients:
1/4 cup poppy seeds
1 cup milk
2/3 cup unsalted butter
1-1/2 cups (300 grams) granulated sugar
1 tablespoon baking powder
3 cups (480 grams.) sifted cake flour (If you only have AP flour, mix 2 1/2 cups and 2 Tablespoons (336 grams) of AP flour with 1/4 cup and 2 Tablespoons (41.25 grams) of cornstarch)
1 teaspoon salt
4 large egg whites, stiffly beaten

Directions:
1) Add poppy seeds to milk and let stand for at least 1 hour (I usually soak them overnight or all day.)
2) Preheat oven to 375 degrees F. Grease and flour a 9 x 13-inch pan. Set aside.
3) Cream butter and sugar until light and fluffy.
4) Whisk flour, baking powder and salt together in another bowl.
5) Add flour and milk alternately to creamed mixture, beginning and ending with flour.
6) Beat egg whites until stiff peaks form. (NOTE: Whip the egg whites right before putting them in. If you let them sit for too long, then they'll probably separate. I've done it. Not pretty. See this place for the science behind it, and for tips about how to make sure your egg whites don't separate--things like adding acid in the form of lemon juice or cream of tartar work nicely).
7) Fold in egg whites, half at a time. Pour into prepared pan. Bake for 30 minutes or until a toothpick inserted in several areas comes out clean. Let sit in pan for 10 to 15 minutes before removing to a rack to complete cooling.

Cream Cheese-Sour Cream Frosting (from here)
adapted from Bunny's Warm Oven where she got it from Epicurious

1 (8 ounce) package cream cheese, room temperature
2/3 cup confectioner's sugar
1/2 cup sour cream
Optional: Walnuts

Beat sour cream, confectioner's sugar and cream cheese in a large bowl until well blended. Spread the frosting over the cooled cake. If desired, sprinkle with chopped walnuts.