A bottle cap is a really good place to start working with carved cake. You don't have to use a pound cake, but if your cake is not firm then you need to use a buttercream that is all butter or ganache. Those will firm up nicely and make the carving easy. For this cake, we baked three layers, a 14", 16" and 18". I didn't carve much to get to this point. I really just filled in with a lot of buttercream (as you can see below). It's easier this way because you don't have all of the soft cut edges to deal with.
I've used a piece of cardboard that is covered on both sides with packing tape. This will allow you to wipe it off a keep it from getting soaked with buttercream. I just make a shape by folding a piece of paper in half and draw a J shape. When you cut it, it should be a mirror of your J, therefore being even and identical on both sides. Don't get the "caves" to close together. The thin piece of cake that will separate them will be weaker the smaller you make it. You can spend hours figuring out the exact space in between each indention so they are all equally spaced or you can wing it and end up with a backside, as we see here. Just remember where the back is so you don't accidently put it in the front.
It's a little hard to see, but I've wrapped a piece of masking tape around my knife as a guide so I don't cut too deep. It's not going to stop you from cutting too deep, it's just there as a visual reference. Now this is the deepest you are going to cut only at the bottom layer and then taper up from there. You want the back wall to be straight-ish.
You have some room for boo boos here. You are going to fill in with a coat of buttercream so if you cut too deep, fill it up with buttercream. You are going to want to use a bag of buttercream and pipe some icing into these caves. Your fingers are going to be the best tools here, but you can also cut a small piece of flexible material like stencil plastic. Don't drive yourself too crazy filling these into be perfect. Get a crumb coat on and chill. Then add a second coat so that your fondant has a nice base to go onto. Icing inside of the caves will take you longer than carving them out. If you can, use some Elite or Fondarific fondant for this project. You need a long time work time to get the fondant into the caves and you need a lot of stretch.
Once you get your fondant applied, you can airbrush or dry brush as darker shade of the fondant inside of the caves to give the illusion of more shadow. Create a template of your favorite brand of soda or beer. Print two copies, one to cut up and one for spacing. Use modeling chocolate and cut out your logo. Arrange all of the cut pieces on top of a print out of the logo so the spacing is perfect. Put Crisco on one side of a piece of wax paper and press it to the front (top) of the cut letters. You should be able to transport the letters from the table onto the top of the bottle cap ensuring the spacing is perfect. I like to use a wet brush a attach the pieces individually, but you could dampen your top side before you place the cut logo pieces on it. cover the top of your cap. Press the pieces down and slowly peal away the wax paper. Poof! Bottle cap!
