Melt butter in a small saucepan over medium heat and add diced jalapeño.
Sauté jalapeño in butter for about 5 minutes, stirring occasionally, to soften. Remove to separate bowl.
Heat coconut oil and 3 popcorn kernels in a large saucepan over medium heat.
When the kernels begin to pop, pour the rest of the kernels into the pot. Cover the pot and continue to cook for about 6-7 minutes, occasionally shaking the pan to keep the kernels moving. When the popping slows, move pot from heat and let stand for another minute.
Put popcorn in a large bowl and toss with jalapeño butter, lemon zest, grated parmesan, and freshly ground black pepper.