Combine lime zest and heavy cream in a small saucepan. Heat over medium heat until cream just starts to simmer. Cover and remove from heat. Allow to steep for 20 minutes.
While cream and lime zest steep, combine white chocolate and butter. Melt in microwave for 45 seconds to get things started. Stir and add salt.
Return cream and zest to medium heat and back to a slight simmer. If you want to remove the zest, pour the cream through a fine strainer onto the white chocolate. If you want the added flavor of the zest, pour the cream directly onto the white chocolate.
Stir until white chocolate is fully melted and the mixture is smooth.
If you want to roll truffles, refrigerate for at least 4 hours before you roll them. Refrigerate or freeze after molding, then melt dark chocolate in microwave in 30 second increments, stirring between rounds, until fully melted but not too hot. Quickly dip truffles in dark chocolate.
If making molded truffles, do not refrigerate truffle filling. Melt dark chocolate in microwave in 30 second increments, stirring between rounds, until fully melted. Fill molds with dark chocolate. Tap molds on counter to get the air out, then pour chocolate out of the molds.
Carefully spoon truffle filling into molds. Keep the level just a little below the top. Refrigerate for 20-30 minutes to start solidifying the filling. Top with more dark chocolate after the filling is sufficiently solid. Tap the molds on the counter to smooth things out, and scrape off excess chocolate if possible.
Refrigerate truffles until the chocolate has fully solidified. They should come out of the molds fairly easily. If they break or leave pieces behind, they need more time in the fridge.