Break the white chocolate into small pieces and add to a medium bowl with the lemon zest. Reserve.
In a small saucepan, over medium-low heat, gently simmer the heavy cream until hot. Do not boil!
Pour the heated cream over the white chocolate & lemon zest. Allow to sit for 3 minutes.
Mix the chocolate mixture with a rubber spatula. If there are any lumps of unmelted chocolate, pop the bowl into the microwave and heat in 30 second increments at 50% power, stirring after each increment. Heat until smooth.
Stir in the lemon juice until completely combined.
Allow the bowl to come to room temperature, and then cover with a layer of plastic wrap. Pop the bowl into the freezer for 2 hours (or longer).
Prepare a large cookie sheet with a layer of parchment paper. Using a small cookie scoop or small spoon, portion out a 1.5 teaspoon portion of chocolate. Gently roll into a nice sphere using your hands. Place on the cookie sheet.
After you've rolled all of your truffles, place the cookie sheet back into the freezer for 30-60 minutes, to set.
After at least 30 minutes in the chiller, you can now prepare your dark chocolate for coating. Break the 6 ounces of dark chocolate into small pieces and place into a heat-safe bowl (I prefer using a metal bowl). Take a small to medium saucepan and fill with about 2 inches of water. Set to simmer over medium heat.
Once the water is simmering, but not yet boiling, set the heat proof bowl over the pot. This is your double boiler.
Stir the dark chocolate with a rubber spatula until completely smooth and melted. Continue to heat, while stirring, for an extra 3 minutes. Remove the bowl from the pot and allow to cool slightly on the counter.
Prepare a second cookie sheet with parchment paper and grab a fork for dipping. I find using a chopstick is also helpful for scooting the truffle off the fork after dipping.
Retrieve the truffles from the freezer.
Take one truffle and gently roll in the melted dark chocolate. Working quickly, using the fork, lift the truffle from the dark chocolate and gently shake the excess off the candy. Next, transfer to the baking sheet, using the chopstick to push the chocolate from the fork.
Repeat with each truffle. If the dark chocolate cools too much and stops being glossy, return the bowl to the double boiler and very quickly (only for about 10 seconds) heat the chocolate again.
After all of the truffles have been dipped, cool them in the refrigerator for 30 minutes.
Keep refrigerated until right before serving; If the truffles have a little pool/foot of chocolate at the base, simply break off with your fingers.