In the same situation in the US, when applying for German Shenghen visa for my mom as a sponsor, I provided the old tax return (for year 2014), a brief informal summary for year 2015 with a copy of the last 2015 paystub (in US it also lists the total income and tax payments), and a note that I haven't filed 2015 tax return yet. It was accepted, and the visa was issued.
If you don't have paystubs showing this information, you can provide either:
- all paystubs for the 2015 (showing you were receiving salary the whole year);
- bank statements, if your salary was deposited to your bank;
- a statement from your accountant summarizing your income for 2015, which should also mention the information where your accountant got those numbers;
If the Consulate needs more information, they would let you know during the interview, and you'd be able to follow up with them (even by email).