The chocolate chips will become liquidy when baked, but if you wait to cut into the brownies until they cool a bit, the chocolate chips will eventually firm up again. If your chocolate chips sink to the bottom of your batter and melt to the bottom of the pan, you can try using a little more flour to thicken your batter, or wait to add the chocolate chips until after you pour the batter into the pan (sprinkle them over top).