That's pretty tricky... It took me almost 30 minutes to get it working right.
The trick is that, to remove 1 character, you ned to write a backspace + space + backspace.... Then when your value decrease from let's say 100 to 99, you need an adjustment so that it "erase" the correct amount of character on the screen... Anyway, here is the working code:
Code:
Module Module1

    Private WithEvents CountDownTimer As System.Timers.Timer
    Private countDownValue As Integer = 1200

    Sub Main()
        CountDownTimer = New System.Timers.Timer()
        With CountDownTimer
            .AutoReset = True
            .Interval = 100
        End With
        Console.Write("Start counting down... " & countDownValue)
        CountDownTimer.Start()

        'Prevent the application to close
        Console.ReadLine()
    End Sub

    Private Sub CountDownTimer_Elapsed(ByVal sender As Object, ByVal e As System.Timers.ElapsedEventArgs) Handles CountDownTimer.Elapsed
        Static lastDigitCount As Integer = countDownValue.ToString.Length
        countDownValue -= 1
        Dim currentDigitCount As Integer = countDownValue.ToString.Length
        'Adjustment to character erasing...
        If lastDigitCount > currentDigitCount Then
            Console.Write(ChrW(8) & ChrW(32) & ChrW(8))
            lastDigitCount = currentDigitCount
        End If

        'Erase the old value and write the new value
        If countDownValue > 0 Then
            For i As Integer = 0 To currentDigitCount - 1
                Console.Write(ChrW(8) & ChrW(32) & ChrW(8))
            Next
            Console.Write(countDownValue)
        Else
            CountDownTimer.Stop()
            Console.Write(ChrW(8) & ChrW(32) & ChrW(8) & countDownValue & Environment.NewLine)
            Console.Write("Press Enter key to exit...")
        End If
    End Sub
End Module