I have just made a banana and poppy seed cake, which has vanilla cream cheese frosting but the frosting is a bit on the runny side. How do I thicken it up ?
The recipe I used is :
70g butter
200g cream cheese
400g icing sugar

Add more icing sugar, do it bit by bit.

I thought about doing that, but I thought it might have become TOO sweet

i can't see it making to much difference, maybe add a bit more cream cheese to even the flavour out

Try using proper cream cheese rather than Philadelphia (or equivalent. It's sometimes called curd cheese and is much thicker.

Or if you're using low fat, make sure you're using full fat.

Give it an hour in the fridge to see if that helps. If you have any double cream, you can also whisk that up till really stiff and fold into the frosting, or slice the cake and fill instead of topping it.

Never had any problem with using full fat philadelphia, mind you ive used full fat tesco & asda and the frosting was runny so always use philadelphia.

Beat/whisk it within an inch of its life......
and/or what Smokey said.

I had this problem making some frosting for carrot cake and found adding more icing sugar just made it even more runny for some reason. I've found the best way is to make a very thick butter icing using the butter, icing sugar and a bit of milk to loosen it enough to mix, then add the cream cheese bit by bit at the end until its the right consistency. Might not be as cheesy as it could be but its the only way I've found to make it thick enough.