Line the base of a 9-inch tart pan with a ring of parchment paper.

Lightly crush the Rice Krispies with your hands until about half remain whole and half are broken into smaller pieces.

Melt the chopped dark chocolate and butter together in a heatproof bowl set over barely simmering water until smooth.

Add the Rice Krispies and salt to the melted chocolate mixture and stir until evenly coated.

Transfer the mixture to the prepared tart pan. Using a spatula, spread it into an even layer.

Press the mixture across the base and up the sides of the pan. Check for any gaps or thin spots and patch them as needed.

Refrigerate for 20–30 minutes, or until firm.

Place the milk chocolate, dark chocolate, and vanilla in a medium heatproof bowl.

In a medium saucepan fitted with a candy or instant-read thermometer, combine the milk, cream, and salt. Heat over medium heat until steaming.

Meanwhile, place the egg yolks in a medium bowl and whisk until smooth.

Slowly drizzle a couple ladles of the hot milk into the egg yolks, whisking constantly.

Pour the tempered yolk mixture back into the saucepan. Cook over medium-low heat, stirring constantly until the custard reaches 175°F (79°C).

Immediately pour the hot custard over the chocolate mixture and let sit for 1 minute. Stir slowly until completely smooth.

Strain through a fine-mesh sieve into the chilled crust set on a sheet pan.

Refrigerate for at least 6 hours, preferably overnight.

Once chilled, remove the tart from the pan, top with meringue if desired, and slice with a hot knife.