Preheat oven to 325 degrees F (165 degrees C). Grease a 10x14-inch baking pan with butter.
Mix pecans, dark chocolate, 3/4 cup sugar, and cinnamon in a bowl. Trim phyllo dough to fit prepared baking pan. Layer 8 sheets of phyllo dough in prepared baking pan, brushing each sheet with melted butter; sprinkle with 2 cups pecan mixture. Spread butter on 4 more sheets of phyllo and layer over pecans; sprinkle with 2 more cups pecans. Repeat layers with 4 more sheets of phyllo and remaining pecans. Pour any leftover butter over baklava.
Cut through the top few layers of phyllo lengthwise, leaving bottom layer intact, to create 4 long rows. Make evenly-spaced zigzag cuts along each row to create small triangles, again leaving bottom layer of phyllo intact.
Bake in preheated oven until phyllo dough is golden brown and flaky, about 1 hour; immediately finish cutting through to bottom layer of baklava.
Bring orange juice, honey, water, 1/2 cup sugar, and lemon juice to a boil in a saucepan; reduce heat to medium and simmer until thickened, about 20 minutes. Pour sauce over baklava.
Melt remaining 2 ounces chocolate and 1 teaspoon butter in the top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching. Drizzle chocolate over baklava. Cover and refrigerate until ready to serve.