Combine butter and 1/2 tablespoon oil in a saucepan over medium-low heat; cook and stir until butter is just melted, 1 to 2 minutes. Stir sumac, chilli powder and salt into butter-oil mixture.
Heat 2 tablespoons oil in a large saucepan over high heat. Add 3 popcorn kernels to the oil and cover saucepan with a lid. Once the 3 kernels have popped, the oil is ready. Add the remaining popcorn kernels, cover saucepan and cook popcorn, shaking saucepan constantly, until you no longer hear popping. Remove saucepan from heat.
Transfer popcorn to a large heat-proof bowl and let sit for 2 minutes to ensure better coverage by the butter.
Pour half of the butter mixture in a second large bowl and swirl it around the bottom and sides; add popcorn and mix with a big spoon. Transfer the popcorn back in the first large bowl; stir again. Pour remaining half butter mixture into the second bowl and swirl around again; add the popcorn in and mix again.