Grind the seeds in a blender, coffee grinder or with mortar and pestle until most of the seeds are broken up. (Seeds can be used whole but the texture and flavor will not be as fine or rich.) Set aside.

In a medium bowl at medium speed, cream butter with sugar, then mix in egg yolks and vanilla. Add flour, baking powder and mix will until combined and crumbly. Firmly press the crumbs into greased 9-inch-square baking pan. Set aside.

In a clean bowl with clean beaters, beat egg whites with the cream of tartar until frothy.  Gradually add brown sugar until stiff peaks form, then fold in the poppy seeds. Evenly spread the meringue mixture over the dough in the pan.

Bake in a preheated 350-degree oven for 30 minutes, or until the top is set and very lightly browned. Cool completely in pan on a wire rack. When cool, cut into 4 large squares, then diagonally cut each square into 4 triangles (to resemble hamantaschen).