How to Make It
Make the vanilla custard sauce: In a medium saucepan, combine the milk and cream with 1/3 cup of the sugar and the vanilla beans and seeds and bring to a simmer over moderate heat. In a medium bowl, whisk the egg yolks with the remaining 1/3 cup of sugar until smooth. Gradually whisk in one-third of the hot milk, then whisk back into the remaining warm milk in the pan. Cook the sauce over moderate heat, stirring gently with a wooden spoon or rubber spatula, until thickened, 2 to 3 minutes; do not let it boil. Strain the sauce through a fine sieve set over a stainless steel bowl. Set the bowl in a slightly larger bowl of ice water and let the custard sauce cool, stirring occasionally. Cover and refrigerate.
Make the praline: Preheat the oven to 350°. Line a rimmed baking sheet with parchment paper. Spread the almonds and hazelnuts in another rimmed baking sheet and toast for 8 to 10 minutes, until lightly browned. Keep the oven on.
In a medium saucepan, combine the sugar with a little water until it looks like wet sand. Cook over moderate heat, stirring occasionally, until the sugar melts, then cook, swirling the pan occasionally, until a light amber caramel forms. Add the toasted nuts and stir to coat. Scrape onto the prepared baking sheet and let cool until hard, at least 10 minutes. Break the praline into small pieces. In a food processor, pulse the praline until ground to a medium-fine powder.
Make the meringue: Line a 9-by-13-inch baking pan with parchment paper. In a stand mixer fitted with the whisk, whip the egg whites at medium speed until frothy. Gradually add the sugar and whip at high speed until the whites hold a medium peak. Spread the meringue in the prepared dish and bake for about 25 minutes, until puffy and just set. Let cool slightly.
Pour the vanilla custard sauce into shallow bowls. Cut the meringue into 8 rectangles. Using a spatula, transfer the meringues to the bowls, sprinkle with the praline and serve.