Prewarm the oven to 177ºC (350ºF), rack in the middle.
Lightly butter 27cm x 20cm (11″x 8″ ) baking pan or line with parchment paper.
Using a hand held electrical mixer and a medium size bowl beat the butter at high speed until smooth, about 1-2 minutes.
With mixer still running at high speed add and beat in one yolk at the time until all 5 are incorporated and mixture is light and fluffy, about 2 minutes.
Slowly add and beat in the sugar.
Using clean beaters, beat the egg whites in another medium size mixing bowl. Run the mixer at high speed until soft peaks are formed.
Using a large silicon spatula stir about 1/4 of beaten whites to the butter mixture to lighten it up. Fold in the rest of the beaten egg whites, stopping just before all the whites are fully incorporated.
Fold in the ground poppy seeds first and then heavy cream.
Pour and spread the batter evenly in the baking pan.
Bake until the top of the cake is lightly browned and the center springs back when gently pressed, about 52 minutes.
Transfer to the wire rack and let cool completely.
Before serving dust each square with confectioners sugar. Serve with a dollop of whipped cream.