In a heatproof bowl, whip the yolks, granulated sugar, and flan powder until smooth. In a small saucepan, combine the milk and vanilla seeds and bring to a simmer; remove from the heat, add the lavender sachet, cover, and rest for 3 minutes. Remove the sachet, return to a simmer, and while whisking, gradually pour half of the hot milk into the egg yolk mixture. Gradually whisk the mixture back into the saucepan. Return the saucepan to medium heat while whisking. Bring to a simmer for about 1 minute, until thickened, to make a pastry cream. Transfer the mixture to a bowl, cover the surface with plastic wrap, and refrigerate until chilled.