COM exe out-of-process servers are notably difficult to write, but Microsoft has created COM+ Component service to ease this out.

It contains a lot of services, but here we're interested by the Application service that allows you to host in-process servers (DLL) in an out-of-process surrogate host.

It's quite simple, just write a standard ATL DLL (or use any other language/framework you like). I recommand using Automation types for the interface so you don't need special proxies, for example with an IDL interface defined like this:

interface ISharedMap : IDispatch{
    [id(1)]
    HRESULT PutData([in] BSTR key, [in] VARIANT value);

    [id(2)]
    HRESULT GetData([in] BSTR key, [out, retval] VARIANT *pValue);
};

Then create a new COM+ application, as described here: Creating COM+ Applications, and declare it as a server application. This is what you should see once this is done:

Your DLL will now be hosted automatically in a specific process (the famous dllhost.exe) which will be started as soon as clients try to connect. By default, the same process will be used for various out-of-process COM clients. It will shutdown after some time, but you can configure the COM+ application in a various ways, for example with the 'Leave running when idle' flag set':

Now you will be able to use your cross process memory cache for all COM clients you'll like, for example, like this from a simple javascript .js code:

var map = new ActiveXObject("SharedMap");
map.PutData("mykey", "mydata")
var data = map.GetData("mykey")

Note: implementation of the cache is left to the reader, but it could reuse another of COM+ services: the COM+ Shared Property Manager