Heat the oven to 163 C / 325 F. Line a 33 x 46 cm / 13 x 18-inch sheet pan with parchment paper, leaving a little overhang on two sides.
In a large mixing bowl, combine the rolled oats, sliced almonds, pumpkin seeds, cinnamon, and fine sea salt.
In a small saucepan over low heat, warm the maple syrup and coconut oil until the oil melts, about 2 minutes. Stir in the vanilla extract.
Pour the warm maple mixture over the oats and stir until every oat and almond slice is coated and sticky.
Spread the mixture onto the prepared sheet pan. Press it firmly and evenly with the back of a spatula into a layer about 6 mm / 1/4-inch thick, edge to edge.
Bake for 22 to 25 minutes, until the edges turn deep golden brown and the center feels set when you press it lightly.
Slide the pan onto a rack and let the bark cool completely, about 20 minutes. Don't move or break it while warm, it will crumble instead of holding together.
Melt the dark chocolate chips in 20-second bursts in the microwave, stirring between each, until smooth. Drizzle over the cooled bark and sprinkle with flaky sea salt.
Chill the pan for 10 to 15 minutes, until the chocolate sets hard.
Lift the whole slab out using the parchment overhang and break it into rough, uneven shards with your hands.