Preheat oven to 350°F (177°C). Line or grease a 9x9-inch (23x23 cm) baking pan and set aside.

Prepare the butter-sugar mixture for the brownies: In a bowl, cream together 3/4 cup (170 g) unsalted butter and 1 1/4 cups (250 g) granulated sugar. Heat briefly in the microwave in short bursts (or warm in a small pan) until the sugar dissolves and the mixture becomes glossy; allow to cool slightly.

In a separate large bowl, whisk the eggs. Add 2/3 cup (56 g) cocoa powder, 2 tbsp (16 g) milk powder, 1/2 tsp instant espresso powder, 3/4 tsp baking powder, 1/2 tsp salt, and 2 1/2 tsp vanilla extract to the eggs and mix until combined.

Combine the egg mixture with the cooled butter-sugar mixture and mix until homogeneous.

Gently fold in 1 cup + 2 tbsp (140 g) all-purpose flour. Mix just until incorporated — do not overmix to avoid a cakey texture.

Fold in 3/4 cup (130 g) chocolate chips.

Pour the batter into the prepared 9x9 pan, smoothing the top. Bake at 350°F (177°C) for 30–33 minutes, until a toothpick inserted into the center comes out with a few moist crumbs but not wet batter. Remove from oven and let the brownies cool in the pan while you make the caramel.

Make the espresso caramel: In a medium saucepan over medium heat, combine 1/2 cup (113 g) unsalted butter and 3/4 cup (150 g) brown sugar. Cook, stirring, until the butter melts and the sugar dissolves and the mixture is smooth.

Add 10 oz (about 285 g) evaporated milk and 2 tbsp (30 g) heavy cream to the pan. Continue cooking the mixture, stirring frequently, for about 10–20 minutes, until it thickens slightly and becomes a bit darker in color. (Adjust time as needed for desired thickness.)

Remove the caramel from the heat and stir in 1 tsp vanilla extract, 1 1/4 tsp instant espresso powder, and 1/2 tsp sea salt. Let the caramel cool for a few minutes off the heat.

When the brownies have cooled to room temperature, pour the warm-but-not-piping-hot espresso caramel evenly over the brownies.

Transfer the pan to the refrigerator and chill for at least 30 minutes to set the caramel (longer chilling will yield a firmer caramel). Before serving, sprinkle flaky sea salt over the top.

Slice into squares (a 9x9 pan yields about 9–12 brownies) and serve.