Place the dark chocolate in a heatproof bowl. Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer. Pour the hot cream over the dark chocolate and let it sit for one minute.

Stir the mixture until the chocolate melts completely and forms a smooth ganache. Stir in the vanilla extract. Cover and refrigerate for one hour until the ganache firms enough to handle.

Scoop the ganache into 8 equal portions and roll each into a ball between your palms. Place the balls on a parchment-lined tray and return them to the refrigerator for 15 minutes.

Melt the milk chocolate in a microwave-safe bowl in 20-second bursts, stirring between each until smooth. Roll each chilled truffle in the melted milk chocolate to coat, then place it back on the tray.

Dust each coated truffle lightly with cocoa powder. Press two sliced almonds into the sides of each truffle to form bat wings. Refrigerate again for 10 minutes to set the coating.