Cream butter and oil in the bowl of a stand mixer for 2 minutes, on medium speed. Gradually add sugar and cream for 3 minutes until mixture is fluffy and lighter in color.
*I like to soak my cakes with simple syrup. To make a simple syrup, just bring equal amounts of sugar and water to a boil until the sugar melts. And then wait for the syrup to cool down. Brush some syrup on top of the cake before filling and frosting it. For a cake this size I’d say making a syrup with 1 cup of sugar and 1 cup of water would be enough.
*Any leftover key lime pie filling will store nicely in the fridge for up to 2 weeks and in the freezer for a couple of months. Roll the leftover up into fudge balls, like here, and place them on top of the cake. Or eat it with a spoon.