Look for almond meal in the baking section or, if shopping at Trader Joe's, in the nut and dried fruit section of your grocery store. If you can't find it there,
it's available on Amazon, or you can make your own by grinding almonds in a food processor (be sure to stop short of making nut butter).
For the chocolate, I used one square (1/2 ounce) of a
Ghirardelli Bittersweet Chocolate 60% Cacao Baking Bar; use whatever dark chocolate you like/have on hand.
The exact volume of the ramekin is less crucial here than the interior diameter (aim for something similar to 2 3/4-inch), as that'll effect the cooking time more than the amount of headspace. I used a 5 1/2-inch ramekin, but larger could work if it has a similar diameter. If your baking vessel is larger or smaller, adjust the cook time accordingly (more for smaller, as the cookie will be thicker, less for larger, as it'll be thinner).