Thanks for telling us where to find the recipe! It sounds delicious, but very rich. It’s like a cake recipe with the flour considerably reduced, and dates & nuts added.
Several reviews on allrecipes.com suggest that you should reduce the butter, and maybe the sugar as well, so that sounds like something to try.
Perhaps the butter is coming out of emulsion? Try being very careful to have the butter and eggs at cool room temperature—not too hot and not too cool. And cream the butter and sugar thoroughly before adding the other ingredients. Beat in the other ingredients gradually, not adding too much all at once.
You could also try the “high-ratio” method of mixing—the one Rose uses throughout the “Cake Bible.”
Any other ideas, folks?