It's probably no surprise that because of my butter obsession, I branched out to things butter-related.
One of my favorite baking ingredients is buttermilk (and yes, the UK buttermilk is out this world compared to the US stuff). Buttermilk helps improve the texture of some baked goods. The acidity in the buttermilk reacts with baking soda (be sure there is baking soda in the recipe if you're using buttermilk) and releases carbon dioxide into the batter. This CO2 creates a fluffier texture, vs. a dense texture. A good example is scones. Some scones can be brick-like, while others tend not to be. Those that aren't have something acidic in the recipe reacting with the baking soda to create a fluffier texture (= better texture, at least in my book).
Another essential ingredient that i'm very familiar with is buttercream. I ran a very small organic cupcake business out of my home when I was living in California. I became well acquainted with your basic buttercream, as well as the more complicated ones (Italian Buttercream being my favorite - it's GORGEOUS!). There's nothing quite like a great frosting to go on top of a wonderful cupcake or cake (or to be eaten just out of the bowl, who are we kidding??). Since relocating to the UK, I haven't made too many baked goods requiring frosting, so i'll be interested to see how much better the frosting is here due to the better quality butters available. We shall see!