In a blender or food processor puree cilantro and jalapenos until smooth. Set aside.
In a separate large bowl, place masa harina, cornmeal, flour, baking powder, sugar and salt together and stir to combine.
Next, in a small bowl, whisk half and half, sour cream, eggs and melted butter together.
Add wet ingredients to dry ingredients and stir to combine. Take care to not over-mix.
Then, remove 1 cup of the batter and place into a separate bowl. Add the pureed jalapeno/cilantro mixture and stir to combine. The batter should be nice and green.
Next, fold the cheddar cheese into the remaining plain batter.
Spray or grease a 9×13 pan or 10-inch cast-iron skillet and spoon the plain batter into the pan and smooth the top. Now, take spoonfuls of the green batter and put on top of the plain batter. Make sure some yellow is still peaking through. Then, make it pretty! Using a knife, swirl the two batters together until well-marbled.
Bake for 20 to 25 minutes—or until the edges are browned and a toothpick inserted into the center comes out clean.
Cool on a rack slightly before serving. Serve warm.