private void Button_Click(object sender, RoutedEventArgs e)
{
    //Variante a) not working:
    Task t = CreateTestFile();
    t.Wait();

    //Variante b) working:
    Task t2 = Task.Run( () => CreateTestFile());
    t2.Wait();
}

private async Task CreateTestFile()
{
    IStorageFolder rootPath;
    if (!(await FolderExists(ApplicationData.Current.LocalFolder, "Test")))
    {
        rootPath = await ApplicationData.Current.LocalFolder.CreateFolderAsync("Test");
    }
    else
    {
        rootPath = await ApplicationData.Current.LocalFolder.GetFolderAsync("Test");
    }
    var file1 = await rootPath.CreateFileAsync(DateTime.Now.ToString("HHmm") + ".txt");
    return;
}

public async Task<bool> FolderExists(IStorageFolder rootPath, string foldername)
{
    try
    {
        await rootPath.GetFolderAsync(foldername);
        return true;
    }
    catch
    {
        return false;
    }
}

private async void Button_Click(object sender, RoutedEventArgs e)
{
   String testFile = await CreateTestFile();
}


private async Task<String> CreateTestFile()
 {
     IStorageFolder rootPath;
     if (!(await FolderExists(ApplicationData.Current.LocalFolder, "Test")))
     {
         rootPath = await ApplicationData.Current.LocalFolder.CreateFolderAsync("Test");
     }
     else
     {
         rootPath = await ApplicationData.Current.LocalFolder.GetFolderAsync("Test");
     }

     var testFile = DateTime.Now.ToString("HHmm") + ".txt";

     var file1 = await rootPath.CreateFileAsync(testFile);

     return testFile ;
 }