Preheat the oven to 325°F (160°C).
Spread the nuts in a single layer on a rimmed baking sheet. Bake for 5–8 minutes until lightly toasted and fragrant.
Line an 8×8-inch baking pan with parchment paper and set aside.
In a large mixing bowl, combine the toasted nuts and puffed rice.
Add the brown rice syrup to a microwave-safe bowl and microwave for 30 seconds to loosen it. Whisk in the vanilla extract and sea salt.
Pour the syrup mixture over the nuts and puffed rice. Stir until evenly coated.
Transfer the mixture into the prepared pan and press firmly into an even layer.
Bake for 18–20 minutes. Remove from the oven and let cool for about 30 minutes.
Transfer the cooled mixture to a cutting board. Slice in half lengthwise, then cut each half into 5–6 bars.
In a microwave-safe bowl, combine the dark chocolate chips and coconut oil. Microwave in 30-second intervals, stirring between each interval, until smooth and melted.
Dip the bottom of each nut bar into the melted chocolate and place onto a parchment-lined tray.
Drizzle extra melted chocolate over the tops and sprinkle with flaky sea salt if desired.
Refrigerate until the chocolate is fully set, then serve and enjoy.