I posted a tutorial on the other site for this, just before it shut down. But since then, I’ve made two more. So here’s the tutorial, plus some extra yarny eye candy! 
This is the yarn I chose for the first jar cake I made, and you’ll soon see why it’s called that.
Here is the jar that is going to be the base for the cake. I’ve seen amigurumi ones, but I CAN’T stand amigurumi and I wanted this to be useful, so I came up with the idea of using a jar.
This is what the bottom of the “cake” looks like. I just eyeballed how long of a chain I needed to make and then joined the ends with a slipstitch. I used half double crochet around. For the “frosting” on the top and bottom, I: half double crocheted, double crocheted, triple crocheted, double crocheted, half double crocheted and slip stitched into the next two spaces and started again. I also did this around the “frosting” on the jar lid, around the outside.
To make the frosting, I just crocheted a flat circle till it was the same size as the lid.
To give the frosting some depth, what I did was: I didn’t cut the yarn and knot it when I was done crocheting the circle. Instead, I started HDC in the top of the next stitch, and did that around and around and around (it’s natural, you’ll see when you do it, where to go next to make a nice spiral). You can use different stitches to get different heights of the frosting, and it looks best (IMHO), when the center part is a little higher than the rest.
This is the frosting for the top of the cake. I used E-6000 (I love that stuff, works on everything!) to glue it on to the jar top and it stuck immediately.
I thought I might add candles, crocheting around pipe cleaners or something. However, I realized that they would get in the way of trying to open the jar. So on the very top of the center, I hand sewed on, with needle and thread, some fancy buttons (on this jar cake, I used strawberry buttons).
This is the finished cake. I did glue on felt eyes and a mouth b/c hubby wanted it, but I like it without.
I made one for one of my best friends, using a cupcake yarn cake (called cupcake b/c it’s smaller) aptly named ‘mint chip’ (my favorite ice cream flavor). Bright lime green is her favorite color so this was perfect!
The yarn cake came with a green pom pom on top (it was intended to be used to make a hat), so I used it on top of the cake.
Lastly, here’s a photo of all three of the cakes I’ve made so far. The purple one is for my other best friend. I used ice cream cone buttons on the top of hers. I’m going to put gift cards and candy in them for their birthdays, which are coming up. These are so much fun to make, use up scrap yarn, and come together quickly.
and such nice gifts for the friends 
