Preheat oven to 350 degrees. Using ½ tablespoon of the butter, grease a shallow baking sheet.
Arrange the oatmeal, pecans, coconut, walnuts, pistachios and sunflower seeds in an even layer on the baking sheet. Bake in the oven for 10 minutes, gently stirring halfway through cooking. Remove from the oven and lower oven temperature to 300 degrees.
Transfer the toasted ingredients to a large bowl. Regrease the baking pan with ½ tablespoon of butter. Set aside.
Melt the remaining butter in a saucepan over medium heat. Stir in the honey and brown sugar and stir until dissolved. Increase heat until the mixture starts to bubble. Remove from heat and stir in the vanilla paste.
Toss the chia seeds in the bowl with the toasted mixture. Drizzle the butter and honey mixture over the granola and stir to combine well.
Press the granola mixture into the prepared baking pan. Using a spatula, press the mixture into an even layer about ½ inch to ¾ inch thick.
Sprinkle the dark chocolate chunks over the top and gently press them into the mixture.
Bake the granola bars for 25 minutes. Remove from the oven and place the baking pan on a wire rack and cool for 3 hours.
Cut into squares.
Store in an air tight container at room temperature.