Took me about 4 hours in inkscape (I wanted it to be completely symetric about the center, if you rotate it 90 degrees it looks like nothing happens).
Download it as an SVG if you want to resize, change color, or rotate it. If you don't have an svg viewer / editor (like inkscape), just right click on the fullsize preview and click save as.
Please tell me if you are going to use it for anything.
Portal and Aperture Science are copyright of Valve.
The cake is a lie.