Lemon Cream:
Place the egg yolks and sugar in a heavy-bottomed, medium saucepan and beat with a medium whisk until all the sugar is dissolved, at least 5 minutes! Stir in lemon juice.

Add flour (or cornstarch) a little at a time to the egg mixture and beat well. Add 1/4 cup milk, mix well. Slowly add remaining milk, stirring continuously until well mixed.

Place pan on low to medium heat and bring to a low boil, stirring constantly. Let boil gently for 1 minute. Remove from heat and stir in butter. Let cool for a couple of minutes, then place a sheet of plastic wrap directly on the custard so it will not form a skin as it cools. Chill.

Just before serving, spoon cooled pastry cream into parfait glasses alternating with berries. Decorate with fresh berries. Dust with powdered sugar. Serve immediately.