For stainless to carbon steel welds, forget about 'buttering'. Use an inconel or incolloy filler instead.
Do not use dissimilar materials in regions of high temperature if you can possibly avoid it: The interface is a mathematical singulariy; ie infinite elastic stress. In practical terms this means cracking along the interface or heat affected zones is just a matter of time and may well be immediate. You might not notice if the cracks arrest themselves but heat transfer will be affected as will fatigue life and potential leaks onto your sensitive electrics. This is an ongoing problem in nuclear fusion reactors where the materials that can be used for divertors are limited so they end up with a brittle tungsten or beryllium armour and copper-based heat sink layers. This is still a subject of ongoing research.
For heat sinks/cooling you might try a compliant interface material such as copper wool or why not even liquid metal like lead (hot zones of large parts) or galinstan (for electronic components) :
"Liquid metal has two major advantages when cooling high power density heat sources: Firstly it has superior thermo physical properties that decrease temperature — and temperature non-uniformity — on die and across chips. Secondly, the electrical properties of the liquid metal enables efficient, reliable and ultra compact electromagnetic pumping without the use of moving parts, shafts, seals, etc."
Of course another advantage is the lack of interface thermal stress.