In a small bowl, combine the poppy seeds & milk. Set aside at room temperature for 1 hour to soften slightly.
Preheat oven to 350 degrees. Butter, flour and line three 9-inch cake pans with parchment paper.
Cream the butter in the bowl of a tabletop mixer fitted with the flat beater. With the mixer running on medium speed, gradually add the sugar. Increase the speed to high and beat until light and fluffy, about 1 minute. With the mixer on medium speed, add the egg yolks one at a time, waiting until each is incorporated before adding the next. Blend in the vanilla.
Sift together into a medium bowl the flour, baking powder and salt. With the mixer on medium-low speed, alternately beat in the dry ingredients and poppy seed mixture in thirds, waiting until each addition is incorporated before adding the next.
In another bowl, whip the egg whites until stiff but not dry. Gently fold the egg whites into the batter. Pour the batter into the prepared pans.
Set the pans on a baking sheet & place on the center rack in the oven. Bake for about 30 to 35 minutes, or until the cake springs back when gently pressed and a toothpick comes out clean when inserted in the center.
Let cool completely in the pans on wire racks, then depan. The cake may be stored, well wrapped, in the refrigerator for 2 days or in the freezer for 2 weeks. Assemble as recipe directs. Makes one 9 inch 3 Layer Cake