Preheat the oven to 350° F. Add the butter to a small skillet set over medium heat. Allow the butter to brown lightly until it smells toasted, about 2-3 minutes. Stir often. Remove from the heat and stir in the sugar, brown sugar, vanilla, and espresso powder, if using. Let this cool until no longer hot, about 10 minutes. In a bowl whisk 1 large egg for 1 minute until bubbly on top. Stir the browned butter mix into the egg. Add 1/3 cup flour and the baking powder. Sprinkle 1 tablespoon flour over the dough. Add the chocolate chunks and mix until combined. Divide the dough between 2 (8-12 ounce) ramekins and bake 18-22 minutes, just until set around the edges, but gooey in the center. Remove from the oven and sprinkle with sea salt. DIG in...preferably with a scoop of ice cream.