create function dbo.ConvertFromBase64String (@Base64Value varchar(max))
returns varbinary(max)
as
begin
    return cast('' as xml).value('xs:base64Binary(sql:variable("@Base64Value"))', 'varbinary(max)');
end;
go

create function dbo.ConvertToBase64String (@BinaryValue varbinary(max))
returns varchar(max)
as
begin
    return cast('' as xml).value('xs:base64Binary(sql:variable("@BinaryValue"))','varchar(max)');
end;
go

declare @b varbinary(max) = cast('hello' as varbinary(max));
select @b; -- 0x68656C6C6F
 
declare @b64 varchar(max);
select @b64 = dbo.ConvertToBase64String(@b);
select @b64; -- aGVsbG8=
 
declare @result varbinary(max);
select @result = dbo.ConvertFromBase64String(@b64);
select @result; -- 0x68656C6C6F
 
declare @t varchar(max);
select @t = cast(@result as varchar(max));
select @t; -- hello