Gather the ingredients. Preheat the oven to 400 degrees F (200 degrees C). Grease a 9x13-inch baking pan.

Beat margarine and sugar together in a large bowl until smooth. Combine cornmeal, flour, baking powder, and salt in another bowl. Gently stir eggs and milk together in a third bowl.

Pour 1/3 of the milk mixture and 1/3 of the flour mixture alternately into margarine mixture; whisk until just mixed. Repeat with remaining flour and milk mixtures until batter is thick enough to not pour easily; stir in jalapeño peppers. Spread mixture evenly into the prepared baking pan.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 22 to 26 minutes. Cool in the pan for 10 minutes before slicing.