Line a baking tray with parchment paper and preheat the oven to 350°F.
In a small mixing bowl, whisk together the flour, cocoa powder, baking powder, salt, and espresso powder. Set aside.
Place the chopped chocolate and butter in a microwave-safe bowl. Microwave for 30 - 45 seconds, making sure to check every 10 seconds and stir the chocolate. It's important to microwave in 10 second intervals so the chocolate doesn't burn. Alternatively, you can melt the chocolate and butter over a double boiler on the stove. Once the chocolate is melted and smooth, set aside to cool.
Use a hand mixer or stand mixer fitted with the whisk attachment, beat together the egg, granulated sugar, and brown sugar until pale and light. You can also do this by hand. Stir in the vanilla extract and melted chocolate and butter. Add the dry ingredients and stir in until just combined.
Fold in the chocolate chunks. Drop 6, one-teaspoon-sized drops of tahini throughout the mixing bowl. Use a butter knife to gently swirl in the tahini, being careful not to fully incorporate it. The mixture should resemble thick brownie batter.
Use an ice cream scooper to place even balls of batter onto the prepared baking sheet, making sure to space each scoop 2 - 3 inches apart. Top each scoop with another tsp of tahini and gently use the butter knife to swirl it in. Bake for 14 - 16 minutes. Remove from the oven and sprinkle with flaky sea salt. Let cool for 10 minutes on the tray.