Hello everyone I could use some help on the using future Methods trailhead. Please keep in mind I am not an Apex coder at all but I am trying to learn as much as I can.

Here is the Task

Create an Apex class with a method using the @future annotation that accepts a List of Account IDs and updates a custom field on the Account object with the number of contacts associated to the Account. Write unit tests that achieve 100% code coverage for the class.
Create a field on the Account object called 'Number_of_Contacts__c' of type Number. This field will hold the total number of Contacts for the Account.
Create an Apex class called 'AccountProcessor' that contains a 'countContacts' method that accepts a List of Account IDs. This method must use the @future annotation.
For each Account ID passed to the method, count the number of Contact records associated to it and update the 'Number_of_Contacts__c' field with this value.
Create an Apex test class called 'AccountProcessorTest'.
The unit tests must cover all lines of code included in the AccountProcessor class, resulting in 100% code coverage.
Run your test class at least once (via 'Run All' tests the Developer Console) before attempting to verify this challenge.

I have written an Apex class and a Test class but I think I am doing them both Wrong: because when I try to check the challenge i Get this error

Challenge Not yet complete... here's what's wrong: 
The 'AccountProcessorTest' test class doesn't appear to be calling the 'AccountProcessor.countContacts' method between Test.startTest() and Test.stopTest().

When I run the Test class I get this error :

System.QueryException: List has no rows for assignment to SObject

Here is the CLASS:
 

public class AccountProcessor {
     @future

  public static void someFutureMethod(List<id> scope) {

   Account[] updates = new Account[] {};
        for (AggregateResult ar : [
                select AccountId a, count(Id) c
                from Contact
                where AccountId in :scope
                group by AccountId
                ]) {
            updates.add(new Account(
                    Id = (Id) ar.get('a'),
                    Number_of_Contacts__c = (Decimal) ar.get('c')
                    ));
        }
        update updates;
    }

}

Here is the Test Class:
 

@IsTest
public class AccountProcessorTest {
  public static testmethod void TestAccountProcessorTest() {
 
Test.startTest();
     Account a = new Account();
        a.Name = 'Test Account';
        Insert a;
      
      Contact cont = New Contact();
      
      cont.FirstName ='Bob';
      cont.LastName ='Masters';
      cont.AccountId = a.Id;
      Insert cont;

    Test.stopTest() ;
     Contact ACC = [select AccountId from Contact where id = :a.id LIMIT 1];

 
        System.assert(Cont.AccountId != null);
        System.assertequals(cont.id, ACC.AccountId);

  }}

I have used alot or diffrent google searches to get me this far. But I have not Idea if Im even remotly close to the right answer or not.