Line an 8-by-8-inch baking pan with parchment paper. Melt together the chocolate, butter and salt in a small saucepan over low heat, stirring occasionally, until smooth, about 10 minutes. Remove from the heat and stir in the vanilla; transfer to a blender or food processor.
Place the egg yolks in a bowl, and whisking constantly, gradually pour in the boiling water. Pour the yolk mixture through a sieve into the chocolate mixture and blend until smooth. Blend in the raspberry jam. Transfer to the prepared pan, spreading evenly and let set, at least 1 hour or overnight.
Place the cacao powder in a small bowl. Remove the truffles from the pan and using a small heart cutter, cut into hearts. Place in the cacao powder and toss to coat. Store at room temperature in an airtight container or refrigerate.