Begin by preheating your oven to 400°F and placing a baking sheet inside to heat up. In a mixing bowl, combine 1⅓ cups (175g) all-purpose flour, 2 teaspoons baking powder, and ½ teaspoon salt.
In a separate bowl, beat together 1 extra-large egg and ⅓ cup (70ml) milk until well combined. Add the wet ingredients to the dry mixture and stir until a soft dough forms. Knead the dough lightly on a floured surface until it comes together, then shape it into a ball.
Roll out the dough on a piece of parchment paper into a 12-inch round, keeping the thickness even throughout.
While the dough rises, prepare the herb butter topping by mixing ½ cup (115g) softened salted butter with 1 large garlic clove that has been crushed to a paste, along with 1 tablespoon chopped parsley, 1 tablespoon snipped chives, 1 teaspoon chopped thyme leaves, and a pinch of sea salt.
Stir until all ingredients are thoroughly combined and the herbs are evenly distributed throughout the butter. Generously spread this mixture over the entire surface of the rolled-out dough.
Carefully transfer the flatbread on its parchment paper to the preheated baking sheet and bake for approximately 12 minutes, until the top is golden and the bottom is crisp. Remove from the oven and slice into wedges while still warm.
Serve immediately to enjoy the flatbread at its best, with the herb butter melted and fragrant throughout.