Grams to tablespoon conversion
It varies by material. A tablespoon (or millilitre) is a unit of volume; a gram of weight. The ratio between the two is called the density, and that varies a lot.

So, you have to look it up, or weight it yourself. Or, if you're lucky, it's on the side of the package.

You can access some of the measurement conversions in the USDA NDB data files I posted a link to via the much-more-friendly web interface. Cocoa powder gives you weight per tablespoon, exactly what you want. Unbleached AP flour gives 125g/cup, and you can convert that to tbsp (since cup is also a measure of volume; Google will happily tell you there are 16 tbsp per cup. (In general, they try to give useful measurements. E.g., "small" and "large" for onions, "stick" for butter, etc.)

From just those two examples, you can see one is 5.4g/T and one is 7.8g/T, so you can't just use one number for all fine dry powders.