Rostizkogel (3375m) from Mandarfen

From the parking lot, start up the ski slope, passing Taschach Alm and heading up the ski slope towards Rifflsee. Once at the lake, either cross the lake (when safe/frozen) or stay near the south shore, then follow the flat valley line into the Riffltal until a short terrain step. Keep the skin track more central and avoid getting too close to the Grubengrat flank.

Climb the step up to around 2400 m, then trend right / NW up a steeper slope with rock islands to a higher terrace. Turn north, move through a broad gully/bowl system, then briefly follow the common track direction toward K2 to a small hill. Continue north over a rib, then NW through a nice bowl aiming for the broken/glacier area of the Nördlicher Löcherferner. 

Around 3000 m, step right out onto the wide glacier and arc left across flatter glacier terrain toward the Rostizkogel summit build-up. The normal way climbs through the broad north ridge, getting progressively steeper. In good conditions, you can also ascent through a steep face towards the east ridge and continue ascending up that ridge. Finish on the narrowing summit crest to the highest point (no summit cross) at 3394 m.

Descent

Normally you ski back along the ascent route to the Rifflsee - huge terrain with plenty of micro-variants. A spicier option mentioned is a direct steep gully shortly after the summit build-up down to the Mittlerer Löcherferner—but this explicitly requires extremely stable conditions. 

From the Rifflsee there’s a short counter-ascent by the lake. Watch out in the spring, the lake can be melting later in the day. Then you can ski the slope back towards Mandarfen for roughly 1600 m of descent total on the day.