In the bowl of your stand mixer fitted with the paddle attachment, beat the peanut butter, butter, granulated sugar and brown sugar on medium speed for about five minutes, until light and fluffy.
Scrape the bowl down and beat in the egg and vanilla.
In a separate bowl, combine the flour, oats, baking soda, salt and cinnamon. With the mixer on low, add the dry ingredients by spoonfuls, just until combined.
Chill the dough for several hours, then preheat the oven to 375. Roll the chilled dough into 3 dozen balls, making sure they're all the same size, and coat each ball in sugar. Place on a parchment lined baking sheet, two inches apart, and flatten slightly with a fork.
Bake until pale golden brown, about 7-8 minutes. Cool for several minutes on the pan, then carefully transfer the cookies to a wire rack to cool completely.
For the ganache, place the chopped chocolate in a bowl. Warm the cream until it simmers, then pour the hot cream over the chocolate. Let stand for three minutes, then stir until smooth and thick. Let thicken slightly at room temperature.
Turn half the cookies over so the bottoms are facing up. Spoon the ganache onto the cookies. Let it set for a few minutes, then carefully place a second cookie on the ganache to make a sandwich. Refrigerate until firm.
Yields about 3 dozen cookies / 1 1/2 dozen cookie sandwiches