For the brownies:
- 1 cup semisweet chocolate chips
- 4 oz unsweetened baking chocolate, coarsely chopped
- 3/4 cup (1 1/2 sticks) unsalted butter, cut into cubes
- 1 1/2 cups granulated sugar
- 3 eggs plus 1 egg yolk
- 1 teaspoon vanilla
- 3/4 cup all-purpose flour
- 1/4 teaspoon fine sea salt
For the cashew butter frosting:
- 4 tablespoons unsalted butter, room temperature
- 3/4 cup creamy cashew butter*
- 3/4 cup powdered sugar
- 1/4 cup milk
- 3/4 teaspoon vanilla
- Coarse sea salt, for topping (optional)
- Heat oven to 350F. Line an 8×8-inch baking pan with foil; spray bottom and sides with cooking spray.
- In a medium microwave-safe bowl, microwave chocolate chips, chopped chocolate and butter on high 1 minute; stir. Continue to heat on high in 15- to 30-second intervals, stirring after each, until mixture is just melted and smooth. Set aside to cool slightly.
- Meanwhile, in a large bowl with an electric hand mixer, beat sugar and eggs + egg yolk on medium-high speed 3 minutes until fluffy and pale. Gently stir in vanilla and chocolate mixture until just incorporated. Fold in flour and sea salt just until a batter forms.
- Pour and spread batter evenly in prepared baking pan. Bake 35-40 minutes until a toothpick inserted in the center comes out with just a few crumbs clinging. Cool completely in pan on cooling rack, about 1-2 hours.
- Meanwhile, make the frosting: In a medium mixing bowl with an electric hand mixer, beat butter and cashew butter on high speed until smooth and combined. Add powdered sugar, milk and vanilla. Beat on medium, then high speed 1 to 2 minutes until frosting is smooth and creamy (add more powdered sugar, 1 tablespoon at a time, if frosting is too thin; add more milk, 1 tablespoon at a time, if frosting is too thick).
- Use an offset spatula to gently frost top of fully cooled brownies. Sprinkle with sea salt, if desired. Use a sharp, clean knife to cut into 16 bars.
- Store brownies in airtight container in refrigerator for up to 3-4 days.
- *NOTE: Feel free to swap the cashew butter for creamy peanut butter or creamy almond butter, if desired! Results may vary.