Butter a 9×13-inch pan and line with parchment, allowing the parchment to overhang the two long sides. Alternately, mist pan with baking spray with flour. Heat oven to 350ºF.

Bring a medium pot with about an inch of water to simmer over low heat. Combine bittersweet chocolate in butter in a heatproof bowl and place over the water. Cook, stirring occasionally, until melted, about 2 minutes. Set aside to cool for about 10 minutes.

Whisk eggs, granulated sugar, brown sugar, bourbon, and espresso granules, and kosher salt into cooled chocolate mixture until well combined. Pour half the batter into the prepared pan. Sprinkle with half of the semisweet chocolate. Pour remaining batter into pan, smoothing the top with a spatula. Sprinkle with remaining chocolate.

Bake until the center is set, about 25 minutes. Remove from oven and immediately sprinkle with flaky salt. Cool in pan for 1 hour. Using the parchment, lift out the brownies then cut into 32 bars.

Once completely cool, brownies can be stored in an airtight container at room temperature.