Fill a medium pot with about an inch of water and bring to a simmer over medium high heat.
In a medium sauce pan or glass or metal bowl (a large enough receptacle to place over pot of water and hold the lemon curd mixture) whisk the egg yolks and sugar, vigorously for about a minute. Add lemon juice and salt, whisking until smooth.
Place sauce pan over simmering pot of water (making sure pan does not touch water). Reduce heat to medium low. Cook mixture, whisking constantly for about 20 minutes until thickened to the point it coats the back of the spoon.
Remove from heat. Add lemon zest and one at a time add each cube of butter, while stirring to combine, letting each cube of butter melt before adding the remaining cubes.
Pour into individual muffin cups. Refrigerate for at least 6 hours or until firm.
Garnish with fresh blueberries and raspberries.