1Preheat the oven to 350˚F. Lightly butter a 9 x 13-inch pan and line with parchment paper, allowing enough overhang on the long side to lift the blondies from the pan.
2Place the butter and 1 cup chocolate chips in a small saucepan over the lowest possible heat and cook until almost all the chocolate has melted. Off heat, stir until smooth. Set aside to cool to room temperature.
3Place the sugar, eggs, and vanilla in the bowl of a mixer fitted with a whisk attachment and whip until lemon-colored and thick.
4Replace the whisk attachment with the paddle and add the flour, salt, and remaining 1 cup chocolate chips and beat on the lowest speed until just combined. Scrape down the sides of the bowl, add the cooled chocolate mixture, and beat until just combined.
5Pour into the prepared pan, smooth with a knife, and transfer to the oven. Bake until the top is just beginning to brown and a tester inserted in the center comes out clean, 20 to 22 minutes. Be careful not to over bake. Cool in the pan slightly and then cut into 24 bars