MD, you could merge your first 2 lines ...

For Each cell In Selection.SpecialCells(xlCellTypeFormulas, 23)
Although, I'd probably just use Len ...

Sub foofoo()
    Dim cell As Range
    For Each cell In Selection.SpecialCells(xlCellTypeFormulas, 23)
        If Len(cell.Value) = 0 Then 
            c = c + 1
        End If
    Next
    MsgBox c
End Sub
We may be able to make this faster by using AutoFilter - if the range is large enough..