Birthday Cake Fudge. Unique fudge #1

Our relief society (church women's group) is having a fudge tasting for our Christmas party.
I'm helping plan and organize.
I'm thinking that chocolate and the "standard flavors will be easy to get people to volunteer to bring. It will be the more unique flavors that will be hard to come by, but necessary for variety.
So I began to search for recipes.

The first that I tried is Cake Batter Fudge
 It original uses a regular yellow cake mix, but I thought I would try it with a GF one.

So my adapted recipe is:
1 Cup of yellow cake mix, gluten free by King Arthur flour.
1 Cup powdered sugar
1/4 cup (half a stick) butter
Microwaved on high for 1 minute. And then it needed 2 tsp of milk, I used coconut.
I tossed in a few gluten free sprinkles, and pressed it into a bread pan.
Then it was chilled for a few hours.

When I took it out, it was WAY to hard. Would not cut, even with a butcher knife swung at a reasonable rate of speed.
BUT after reaching room temperature, it softened enough to slice, but not very clean slices.
Admittedly, not the consistence I would associate with Fudge, but I would not know what else to call it.
Next time there might be a higher butte to cake mix ratio. Because I do have the rest of the box of cake mix...