Butter and eggs must be at room temperature before making cake. If your eggs are cold place them in warm water for a few minutes before starting the process. Adjustments to consider if at a different altitude (I live at 4,200 ft). If at a higher altitude try using all purpose flour for more structure, decreasing sugar (sugar tenderizes), or increasing egg yolks (for structure). Try just one at a time until you get a good balance. If you're at a lower altitude, you'll want to do the opposite as your cake or cupcakes will rise slower - try increasing leavening or sugar and decreasing flour or egg yolks.