I am confused regarding the use of cost method or equity method in the separate financial statements of a parent company to report for investments in its subsidiaries.
The Framework and IAS 1 allow both valuation methods. When company accounts are filed in combination with consolidated accounts it is normal business behaviour to include the investments in group companies/subsidiaries at equity value. This is to avoid that shareholders equity is different in the consolidated accounts as compared to the company accounts. However that is not ruled or prescribed by IFRS/IAS. Company accounts with investments in subsidiaries valued at cost are many times company accounts made for fiscal or other government (special) purposes.
Hope this helps,