Preheat oven to 160 C / Gas 3. Grease a 25cmx36cm baking tin with butter.
Mix pecans, dark chocolate, 150g caster sugar and cinnamon in a bowl; set aside.
Trim filo pastry sheets to fit baking tin. Layer 8 sheets of filo in prepared baking tin, brushing each sheet with melted butter; sprinkle with 1/3 of the pecan-chocolate mixture. Spread butter on 4 more sheets of filo and layer on top; sprinkle with another 1/3 of pecan-chocolate mixture. Repeat layers with 4 more sheets of filo and remaining pecan-chocolate mixture. Pour any leftover butter over the top of the baklava.
Cut through the top few layers of filo 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 filo intact.
Bake in preheated oven until filo pastry is golden brown and flaky, about 1 hour; immediately finish cutting through to bottom layer of baklava.
Bring orange juice, honey, water, 100g caster sugar and lemon juice to the boil in a saucepan; reduce heat to medium and simmer until thickened, about 20 minutes. Pour sauce over baklava.
Melt remaining 55g dark 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.