Broker accounts and mutual funds are required to handle Stock transactions.

Broker accounts are asset accounts, within which you can have funds such as "Stock at Broker", "Money Market at Broker", etc.

You may also have Mutual Funds at a Mutual Fund Company, such as The Vanguard Group. In this case, you would have an Asset Account "Vanguard Group", and within that asset you might have any number of Mutual Funds or Money Market Funds. An example might be "Building Fund at Vanguard Prime MMF"

For any of these funds you will need an Income and Expense account to deposit money and remove it, and additionally and Interest Income Account such as "Interest Income - Vanguard Prime".

You must consider broker accounts like bank accounts. They have to have funds under them, and income and expense accounts to allow you to do the same thing with them that you have to do with the bank accounts.