Preheat oven to 400°F. Put the tablespoon of butter in a small cast-iron skillet and place it in the oven to preheat while you mix the cornbread.
In a medium mixing bowl, whisk together the almond flour, baking powder, and salt until combined.
Add the egg and buttermilk to the bowl and whisk until combined.
Fold in the diced green onions and jalapeño.
Carefully remove the hot skillet from the oven. Transfer the batter to the hot skillet. Use a knife or offset spatula to smooth the top.
Bake until the edges and top are a deep golden color, the center is no longer jiggly, and a knife inserted in the center comes out clean or mostly clean, about 20 minutes.
Remove from oven and let the cornbread rest at room temp for 10 minutes before slicing. Serve warm.