I a large bowl combine masa with cheese,baking soda and salt. Slowly pour the warm water in and stir well. Using your hands,knead until smooth dough forms. Form into 6 even balls,flatten into circles, about 4 inches in diameter.In a cast iron skillet or flat top grill, melt half of the butter on medium heat.
Working in batches,if necessary,cook Arepas until golden brown, about 3 minutes.Add remaining butter,turn Arepas and cook 2-3 minutes until golden. Set aside.
Making the Sandwich:.
Slice the arepas in half horizontally. Layer on the ham,cheese and tomato, season with salt and pepper. Top fixings with remaining Arepas halves. Heat half the butterin a skillet or flat top grill. Cook Arepas in batches,about 3 minutes, until golden. Add remaining butter,turn,cover and cook 2-3 minutes until cheese is melted and Arepasare golden. Serve with Salsa on the side.