Lemon Curd Filling:

Whisk zest and juice, sugar, butter, eggs, vanilla, and salt together in a stainless steel or other non-reactive saucepan over low heat. Continue whisking until curd is thick and starts to bubble. 

Place a piece of parchment paper on top of the cream (so that it touches the cream) to cover the surface and keep a skin from forming. Cool completely.

Fill a pastry bag, fitted with a round tip, with lemon curd. Poke a hole in the side or on top of the Sufganiyot and fill.

Lemon Meringue Topping:

In a medium-size, heatproof bowl, whisk together egg whites and sugar.

Place the bowl over a small saucepan of simmering water, making sure the water doesn't touch the bowl.

Keep whisking the mixture until the sugar has dissolved. You can easily check whether the sugar has dissolved by rubbing a bit of the mixture between your fingers; the mixture shouldn't feel grainy. 

Using a stand or hand mixer, beat the warm mixture until the meringue holds a stiff peak and has cooled to room temperature.

Dollop the meringue on top of the doughnuts in “peaks.” Use a kitchen torch to brown meringue for a bruléed effect, or skip the torch and eat.