How to test asp.net core built-in Ilogger

As @Nkosi’ve already said, you can’t mock an extension method. What you should mock, is the ILogger.Log method, which LogError calls into. It makes the verification code a bit clunky, but it should work:

MockLogger.Verify(
    m => m.Log(
        LogLevel.Error,
        It.IsAny<EventId>(),
        It.Is<FormattedLogValues>(v => v.ToString().Contains("CreateInvoiceFailed")),
        It.IsAny<Exception>(),
        It.IsAny<Func<object, Exception, string>>()
    )
);

(Not sure if this compiles, but you get the gist)