Nonstick cooking spray
1 1/4 cups all-purpose flour
1/4 cup unsweetened cocoa powder
3/4 teaspoon baking powder
1/2 teaspoon kosher salt
1 teaspoon espresso powder, optional
1 stick unsalted butter, cut into pieces
8 ounces bittersweet chocolate, chopped
1 cup sugar
3 large eggs, lightly beaten
1 teaspoon vanilla extract
1 cup chopped walnuts or pecans, optional
1 cup semi-sweet chocolate chips
Lightly coat a 5-quart crock pot insert with cooking spray. Line the bottom and sides of the insert with parchment paper, then lightly spray parchment paper with cooking spray.
Whisk together flour, cocoa powder, baking powder, salt and espresso powder.in a small bowl.
Place butter and chopped bittersweet chocolate in a large microwave-safe bowl. Microwave for 30 seconds and stir. Microwave another 30 seconds and stir again. Continue microwaving in 30 second intervals until melted.
Add sugar to chocolate mixture and whisk to combine. Whisk in eggs and vanilla. Stir in flour mixture until flour is just moistened. Fold in pecans and chocolate chips.
Pour into crock pot, smoothing out the top with a rubber spatula. Cover and cook on LOW 3 1/2 hours. Uncover and continue to cook on LOW for another 30 minutes.
Remove crockpot insert, run a knife around the edges to loosen the brownies. Cool insert on a wire rack for about 2 hours. Remove the brownies from the slow cooker and cut into pieces.
Note: The middle may not look done but will firm up as the brownies cool.