Let me try to clarify the instructions so you can make this successfully. The pumpkin puree is added along with the rest of the list of ingredients (cooled coffee, oil, pumpkin pie spice and eggs), somehow I missed adding it to the instructions.
If the filling frosting is thinner than you would like, add more powdered sugar. Just as in any frosting if you need it thicker add more powdered sugar. Warmer weather will effect how thick/thin frosting will be along with what temperature your butter is at when you begin. The icing on top,has a tablespoon of coffee, white food coloring (which is liquid) and that was plenty of liquid for the glaze to stay in place when I applied it. If you need it thinner ad water If the glaze is to thin it will run down the cake and not stay in place, this was a thick glaze, add liquid a little at a time, sometime I use a dropper to get the perfect consistency I’m looking for, or a spray bottle works well too. I post about glaze icing often and forget that some people are maybe be as familiar with adjustments for it. When making glaze icing or frosting recipes the trick to the perfect consistency you are looking for is in the liquid and powdered sugar, always have it at hand for adjustments. I hope that is more helpful, let me know if you need any further clarifying on the recipe.
Happy Baking!
~Diane

Any chocolate cake recipe would be a good substitute.
Happy Baking!