Almond Cake

I've made this recipe three times now. The first time I couldn't get my whites to stiffen but I didn't want to waste all my ingredients so I threw in my sad whites and added a quarter cup of vanilla yogurt. I don't know why, I just thought it would help. The cake turned out beautiful. The second time I made this cake, my whites stiffened up great. I tried to fold them into the rest of the batter and it was a lumpy mess. Without overmixing, I just put it all in the pan. It turned out good, definitely a little lighter in texture. The third time I tried to recreate my first go at it, so I skipped the stiff whites and used just barely beaten whites. I preferred the cake a little more dense anyway and it for sure wasn't worth the trouble of beating whites to stiffness with a fork (I live in a mixerless home). Also threw in yogurt again because why not. It was exactly perfect.

So I tried the recipe and followed every step on point. However I'm having trouble with the batter. It is very thick and sticky thus the cake comes out hard. I understand there is no oil or water used in this recipe. How do you make your batter more liquid?

This is my cake for every occasion, because of the vast versatility. I typically add a small amount of almond extract and some lemon juice. Sometimes I'll add amaretto, or orange liquor. I've replaced the lemon zest with the zest of another citrus, with good results. Sometimes, I grind whole almonds (toasted or not) to make a more rustic cake with a course texture. Sometimes, I use blanched finely ground almond flour. I've made it for dinner parties, for brunches, or just to have around. I've used it as a base to make a loose version of a Tarte de Santiago. Right now, I'm batch baking several of them to freeze, to have around for summer house guests. They freeze very well.