Is this cake best served the same day made or will a day improve the flavor?

Most cakes that "improve with age" do so in the fridge. 'Ambiant' temperature in most household kitchens is higher than the rest of the house and cakes tend to dry out. The refrigerator is a moist environment that a lot of cakes like. Of course there are a hundred exceptions to every rule, and more for ever cake and their eaters!

It's not going to last that long anyway, trust me

It's really good the day you make it. It also tastes really good the next day though I wouldn't say it improves with age. If it's covered well, it will st taste good for several days