- Place a medium pot filled with an inch of water on the stove and bring to a gentle simmer. In a bowl that fits over the pot but without touching the water, whisk the eggs, sugar, water and salt. Whisk the egg mixture over the simmering water until it doubles in volume and holds a ribbon when the whisk is lifted, about 8 minutes.
- Remove the bowl from the heat and stir in the chopped milk chocolate. Switch to a spatula and stir gently until the chocolate is melted. Allow this chocolate mixture to cool to room temperature, about 20 minutes.
- Using electric beaters, beaters or by hand, beat in the butter until evenly incorporated. In a separate bowl and with clean beaters, whip the cream and vanilla until it holds a soft peak. Fold the whipped cream into the chocolate mixture in 2 additions and spread this into the cooled tart shell. Chill the tart for at least 2 hours before slicing and serving.