Preheat the oven to 350°F. Lightly grease a 9" x 13" pan.
Melt the butter, and stir in the brown sugar. Add the salt, vanilla, and flavor, stirring until well combined.
Allow the mixture to cool slightly (if it's very hot to the touch), then add the eggs one at a time, beating well after each addition. Scrape the sides and bottom of the mixing bowl midway through this process.
Add the baking powder, espresso powder, and flour, stirring to combine.
Stir in the chips or other additions of your choice.
Scoop the batter into the prepared pan, spreading it to the edges with a wet spatula (or your wet fingers). Smooth the top as best you can.
Bake the bars for 30 to 32 minutes, until they've risen, and their top is shiny and golden. Bake for the shorter amount of time in a darker pan; the longer time in a light/shiny pan. A cake tester inserted into the center won't come out clean; in fact, the center of the very middle of the pan may look quite molten when you dig into it. But so long as no wet batter is showing farther out towards the edges, the bars are done. As they cool, the center will solidify.
Remove the bars from the oven, and use a heatproof spatula to press down the risen edges; this will make bars with flat, rather than humped tops. Let the bars cool completely before cutting; overnight is best. Once bars are cool, wrap airtight. Store wrapped bars at room temperature for a couple of days.
Tips from our Bakers
- For old-fashioned "congo bars," use a mixture of your favorite flavored chips: chocolate, white chocolate, cinnamon, cappuccino, butterscotch, etc.; plus chopped walnuts or pecans.
- These bars are very moist indeed just after you've removed them from the oven. You might think they're underbaked; that would be true, if you were going to eat them right away. But letting them cool completely (overnight is best) leaves them with wonderfully chewy, moist texture; if you baked them completely through initially, their final texture would be much drier.
- To regain that delicious just-baked, melty chocolate freshness, reheat individual bars in the microwave very briefly, just until warm.
- To make a crowd-sized batch of these bars:
16 tablespoons (1 cup) butter (8 ounces, 228g)
3 cups + 3 tablespoons brown sugar (24 ounces, 681g)
1 1/2 teaspoons salt
1 1/2 teaspoons vanilla extract
3/8 teaspoon butterscotch or vanilla-butternut flavor, optional
5 large eggs
1 tablespoon + 3/8 teaspoon baking powder
3/4 teaspoon espresso powder, optional
4 cups + 2 tablespoons King Arthur Unbleached All-Purpose Flour (17 1/4 ounces, 489g)
4 1/2 cups chocolate chips (27 ounces, 765g)
Mix everything as directed above. Spread the batter into a half-sheet pan (approximately 13" x 18" outside dimensions). Bake in a preheated 350°F oven for 32 to 34 minutes, or until the bars test done as described in the directions above.