Ah, birthday cake. I love making birthday cakes. Especially for my friends who have very low expectations (if any at all). From figuring out my flavor scheme to the silence that falls upon a room of cake eaters, I love it all. Rarely do my cakes turn out beautifully, but they are usually tasty.
For N's birthday last month I made him a vanilla sugar cake with plenty of lemon flavor. Originally it was supposed to be a layer of cake, a layer of lemon bar filling, and then another layer of cake, but the lemon bar filling had other plans. It was much to runny and gooey to remove from the pan smoothly, so I ended up spooning it on top of each cake layer. If I decided to make a cake like this again I'll probably just make a thinner layer of lemon bar filling and hope it bakes up stiffer. I also regret not making short bread to turn into crumbs to put between the layers. I think it would have added something.
Ratio by Micahel Ruhlman. He says a spongecake should be 1 flour: 1 egg: 1 sugar: 1 butter by weight and I have found that 200g of everything makes 2 8-inch layers. For the lemon bar filling/curd I varied this recipe a bit, and obviously a bit too much since the point of this recipe is that the filling sets, and mine didn't. The buttercream was as basic as it gets (butter, powdered sugar) with a bit of lemon juice and zest for flavor.