In a large bowl using an electric hand mixer or in the bowl of a stand mixer with the paddle attachment, beat butter, sugars and vanilla on medium-high speed until light and fluffy, about 3 to 5 minutes.

Scrape down sides of the bowl, then stir in flour on low speed until just combined. Stir in 6 ounces chocolate chunks and 2 tablespoons orange zest until just incorporated.

Divide dough in half. Place each half on a large sheet of plastic wrap. Fold over plastic wrap to cover dough, then use your hands to roll each piece of dough into a log about 2-2 1/4 inches in diameter. Wrap dough in plastic wrap to cover completely and chill at least 2 hours until firm (can be chilled up to 1 week).

Heat oven to 350 degrees F. Line a cookie sheet with parchment paper. Uncover one log and, using a sharp serrated knife, gently slice log into 1/2-inch-thick rounds (use a sawing motion to cut through chocolate chunks). Place cut side-up on prepared cookie sheet about 1 inch apart. Sprinkle with sea salt, if desired.

Bake 13 to 15 minutes or until edges of cookies are just beginning to brown. Cool 1 minute on cookie sheet, then transfer to cooling rack to cool completely. Repeat with second log of dough.

Melt remaining 6 ounces semisweet chocolate in microwave or in a double boiler according to package directions. Dip one end of each fully cooled cookie into melted chocolate; let excess chocolate drip off, then place cookie on cooling rack. Sprinkle with more sea salt and orange zest, if desired. Chill dipped cookies in fridge 30 minutes until chocolate is set.

Store cookies in an airtight container up to 5 days.