I have a wonderful chocolate cake recipie but it doesn't make quite enough mixture to make my cake the height I want it. I would like it to be about 2 -3 cm higher. could I ajust the mixture to make more.

it depends on the recipe, but yes, you could just double it for example.
xx

Be sure to adjust all components not just one...

You can double the recipe.
Doubling the recipe is always good & with the extra make cupcakes for the kids :0) I do this & freeze the cupcakes for the kids lunches :0)

Doubling a recipe does not always work. I would try it for a home cake before I made it for a client.

I use this article for adjusting cake quantities.
http://www.deliaonline.com/community/yourrecipes/baking/Scaling-cake-ingredients.html
Doesn't help with cooking times obviously but quantities work out pretty well.

Search the recipes here for Cake Extender. Here's the one I use-it increases your batter volume by about 2 cups-you can cut it in half if you don't need that much
1 cup flour
3/4 cup sugar
1 tsp baking powder
1/2 cup vegetable oil
2/3 cup water

Add the dry to your dry ingredients or your dry cake mix - add the oil and water with your other wet ingredients. If you are making a chocolate cake you can take 2-3 T of the flour out and replace it with 2-3 T cocoa powder.

I am assuming your recipe is a scratch recipe. If it is, the extender recipe may not work. Scratch recipes are a chemical reaction between everything.

You may be able to half your recipe instead of doubling it, but definitely do a test run first, as not all scratch recipes will work the same if they are halved or doubled.

If you're using a mix to start with then I would try one of the extender recipes.