How to expand nested list within specific column?

AVS218 - this seems to be a relatively simple if/then/else construct. Why are you embeddding everyting in a list? Just change the Start Year column to this:

if [Count] = 1 then if [Custom] = 1 then [Contract Start Date] else Date.AddYears([Contract Start Date],[Custom]-1) else [Contract Start Date]

If there is some reason you want to keep creating lists just to extract from them, then you could do this:

    #"Start Year (Nested)" = 
        Table.AddColumn(
            #"Changed Type1", 
            "Start Year", 
            each 
                if [Count] = 1 then 
                    {if [Custom] = 1 then {[Contract Start Date]} else {Date.AddYears([Contract Start Date],[Custom]-1)}}{0}{0}
                else {[Contract Start Date]}{0}
            )

But it is the same result, with more work. Don't use squiggly brackets unless you definitely need to create a list. Here, you definitely do no need to do that.