In small saucepan over very low heat, melt both chocolates with the butter, stirring until smooth. Let cool.
In a small bowl, whisk together the flour, baking powder, and salt. Set aside.
In a medium bowl using an electric mixer, beat the granulated sugar and eggs until pale and light, about 2 minutes. Beat in the cooled chocolate, liqueur, and vanilla.
Reduce the mixer speed to low and beat in the flour mixture until blended. Cover the bowl and refrigerate for 1 hour.
Preheat the oven to 325°F. Line 2 baking sheets with parchment paper. Put the confectioners' sugar in a shallow bowl. Roll the dough into 1 ½ inch balls, then generously coat with the sugar. Place the balls, about 2 inches apart, on prepared baking sheets. Bake until just set, 12 to 15 minutes. Let cool on a rack.