Prepare the cake mix according to the package instructions, substituting vanilla-flavored milk for the dairy milk called for on the package. Set aside.

Preheat an oven to 400 degrees. Melt the butter in an 8x8 inch non-stick pan by sticking it in the oven for a few minutes. Check on it frequently to make sure it doesn't burn; it will only take a minute to melt.

When butter has melted. sprinkle the brown sugar on top of the butter and use the back of a spoon to make sure it distributes evenly.

Place the sliced nectarines in a row across the bottom of the pan. Follow with a thick line of blueberries, and repeat the pattern once more so that there are two rows of nectarines and two rows of blueberries, as pictured. Take care to make sure as much of the brown sugar is covered as possible, You wanna avoid lots of holes or gaps.

Pour the cake mixture over the fruit and make sure it's spread evenly. Bake according to package instructions, until cooked completely.

Remove from the oven and immediately turn the pan over on to a serving plate. DON'T LIFT THE PAN UP. Leave the pan right where it is (upside down over your serving dish) for about 5 minutes, just to be sure all the fruit falls from the bottom.

After 5 minutes, lift the pan and set aside. The cake will be very hot and might look a little flatter than the depth of your pan. That's OK; it's not meant to be a super thick cake. Let it rest for another 10 minutes before you cut into it. Serve warm, topped with maple butter.

Combine all ingredients for the butter in a bowl and microwave for 10-20 seconds, until butter is just melted. Stir to incorporate all the ingredients. The result will be liquidy, like a thick syrup. You can definitely serve it just like this, but I like to let it hang out for about 10 minutes at room temp to allow it to re-butterize. After it rests for a few, it'll be a soft, spreadable butter. Yum.