Here's a PowerShell one-liner you can run from a cmd console that'll Base64 encode a string.

powershell "[convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes(\"Hello world!\"))"

It's probably not as fast as npocmaka's solution, but you could set a console macro with it.

doskey btoa=powershell "[convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes(\"$*\"))"
doskey atob=powershell "[Text.Encoding]::UTF8.GetString([convert]::FromBase64String(\"$*\"))"

btoa Hello world!
btoa This is fun.
btoa wheeeeee!
atob SGVsbG8gd29ybGQh

Be advised that doskey doesn't work in batch scripts -- only the console. If you want do use this in a batch script, make a function.

@echo off
setlocal

call :btoa b64[0] "Hello world!"
call :btoa b64[1] "This is fun."
call :btoa b64[2] "wheeeeee!"
call :atob b64[3] SGVsbG8gd29ybGQh

set b64
goto :EOF

:btoa <var_to_set> <str>
for /f "delims=" %%I in (
    'powershell "[convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes(\"%~2\"))"'
) do set "%~1=%%I"
goto :EOF

:atob <var_to_set> <str>
for /f "delims=" %%I in (
    'powershell "[Text.Encoding]::UTF8.GetString([convert]::FromBase64String(\"%~2\"))"'
) do set "%~1=%%I"
goto :EOF

Or if you'd prefer a batch + JScript hybrid:

@if (@CodeSection==@Batch) @then
@echo off & setlocal

call :btoa b64[0] "Hello world!"
call :btoa b64[1] "This is fun."
call :btoa b64[2] "wheeeeee!"
call :atob b64[3] SGVsbG8gd29ybGQh

set b64
goto :EOF

:btoa <var_to_set> <str>
:atob <var_to_set> <str>
for /f "delims=" %%I in ('cscript /nologo /e:JScript "%~f0" %0 "%~2"') do set "%~1=%%I"
goto :EOF

@end // end batch / begin JScript hybrid code
var htmlfile = WSH.CreateObject('htmlfile');
htmlfile.write('<meta http-equiv="x-ua-compatible" content="IE=10" />');
WSH.Echo(htmlfile.parentWindowWSH.Arguments(0).substr(1)));

Edit: batch + VBScript hybrid for @Hackoo:

<!-- : batch portion
@echo off & setlocal

call :btoa b64[0] "Hello world!"
call :btoa b64[1] "This is fun."
call :btoa b64[2] "wheeeeee!"
call :atob b64[3] SGVsbG8gd29ybGQh

set b64
goto :EOF

:btoa <var_to_set> <str>
:atob <var_to_set> <str>
for /f "delims=" %%I in ('cscript /nologo "%~f0?.wsf" %0 "%~2"') do set "%~1=%%I"
goto :EOF

: VBScript -->
<job>
    <script language="VBScript">
        Set htmlfile = WSH.CreateObject("htmlfile")
        htmlfile.write("<meta http-equiv='x-ua-compatible' content='IE=10' />")
        if WSH.Arguments(0) = ":btoa" then
            WScript.Echo htmlfile.parentWindow.btoa(WSH.Arguments(1))
        else
            WScript.Echo htmlfile.parentWindow.atob(WSH.Arguments(1))
        end if
    </script>
</job>

the usual windows way to generate a base 64 string is

Make any file (here simple text but can be PDF, docX or whatever.)

echo Hello World!>input.ext

Convert to base64

certutil -encodehex -f "input.ext" "output.txt" 0x40000001 1>nul

to see the result use

type output.txt
SGVsbG8gV29ybGQhDQo=

To send to another command use a pipe to use the TYPE output.txt as input. However like many conversion tasks it needs to be a file IO. To avoid any hint of a hard drive file, it would need a memory file system.

For looping a folder use a for loop with variables to suit

to decode output.txt

certutil -decode -f output.txt output.ext 1>nul

type output.ext

Hello World!

Answer

There is a very small (7.5 kB) downloadable exe c 2015 that can be used to encode/decode to file or console. You can try to download direct in a curl command like this but it must be 7680 bytes.

>curl -O https://master.dl.sourceforge.net/project/base64/b64.exe?viasf=1
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  7680  100  7680    0     0  12070      0 --:--:-- --:--:-- --:--:-- 12132

>b64.exe

  b64      (Base64 Encode/Decode)    Bob Trower 2001/08/03
           (C) Copr Bob Trower 1986-2015     Version 0.94R
  Usage:   b64 -option [-l<num>] [<FileIn> [<FileOut>]]
  Purpose: This program is a simple utility that implements
           Base64 Content-Transfer-Encoding (RFC1113).
           Use -h option for additional help.

>

Thus use is very simple:

b64 -e input.ext

Result on screen or use in a pipe

SGVsbG8gV29ybGQhDQo=

Note that's exactly the same as above however there has been no update as to most recent query about why it may not always work inbound as expected, but this is an outbound usage.

example of open issue

echo Hello World!|b64 -e

not an accurate result compared to above but an acceptable troughput SGVsbG8gV29ybGQhCg==

so when using

echo SGVsbG8gV29ybGQhCg==|b64 -d

It does correctly report

Hello World!

The reason for the difference is the last EOL
0000000C: 0A 0D
And depending on circumstances, may or may not be a material difference. Also note that command lines are limited in working length, so console usage is also limited for string handling too