Step 1

In a heat-proof bowl that will fit in the inner pot, beat the sugar and butter with a hand mixer until the sugar has mostly dissolved and the mixture is light colored and fluffy. Add the egg yolks and beat until combined. Add the lemon zest, lemon juice, and salt, and beat to combine. The mixture will appear grainy. Cover the bowl with aluminum foil.

Step 2

Add 1 cup of water to the inner pot. Place a trivet with handles in the pot and then place the bowl on top. Lock the lid into place. Select Pressure Cook or Manual, and adjust the pressure to High and the time to 10 minutes. After cooking, let the pressure release naturally for 10 minutes, then quick release any remaining pressure. Unlock the lid. Carefully remove the bowl, and remove and discard the foil. The mixture will appear clumpy and curdled.

Step 3

Whisk the curd mixture until smooth. Place a fine-mesh strainer over a medium bowl and pour the curd through it, pressing down with a flexible spatula to pass the curd through, leaving the zest and any curdled egg bits behind (discard those). Cover with plastic wrap, pushing the wrap down on top of the curd to keep a skin from forming. Refrigerate until cooled, about 2 hours.

Step 4

When the curd is cooled, whip the cream until medium-firm peaks form. Add about a third of the whipped cream to the curd and fold it in gently. Repeat with another third of whipped cream, then finish with the last third. Spoon the mousse into dessert dishes and serve immediately, or refrigerate, covered, up to overnight.