In stainless-steel bowl, whisk together sugar, lemon juice, zest and eggs. Place bowl over saucepan of boiling water. Whisk over heated pot on medium heat until thickened (about 20 min.). Remove from heat and gradually whisk in softened butter, a little at a time. Once all butter is added, cover with plastic wrap and put in fridge to chill. Serve on scones or use it to fill tart shells. Top with summer berries.