Preheat your oven to 330°F and line baking sheets with parchment paper. In a medium bowl, whisk together the flour, cornstarch, baking powder, and salt—this ensures the leavening agent and salt distribute evenly throughout the dough for consistent texture and flavor in every bite.

In a large mixing bowl, beat the softened butter and granulated sugar together for 2-3 minutes until the mixture becomes pale, fluffy, and noticeably lighter in color. This creaming process incorporates air into the dough, which helps the cookies develop a tender, slightly delicate crumb. Add the vanilla extract and mix until fully combined.

Pour the sweetened condensed milk into the creamed butter mixture and stir until fully incorporated—the condensed milk adds richness and acts as both a sweetener and moisture agent. Add the dry ingredient mixture from Step 1 and fold gently with a spatula until just combined. I prefer to fold rather than beat at this stage to avoid overdeveloping the gluten, which keeps the cookies tender rather than tough. Don't overmix; stop as soon as you don't see any dry flour streaks.

Gently fold the semi-sweet chocolate chips into the dough using a spatula, distributing them evenly throughout. Work carefully to avoid breaking up the delicate dough structure.

Using a small cookie scoop or your hands, shape the dough into 20-gram balls (roughly the size of a walnut) and place them about 2 inches apart on the prepared baking sheets. I like to chill the shaped dough balls for 15-20 minutes in the freezer before baking—this helps them hold their shape better and prevents excessive spreading in the oven.

Bake for 14-15 minutes, until the edges are lightly browned but the centers still look slightly underdone—they'll continue to cook as they cool. The cookies should look soft and pillowy when you remove them from the oven; they firm up significantly as they cool on the pan.

Let the cookies cool on the baking sheet for 10-15 minutes; this resting period allows the structure to set without becoming hard. Transfer to a wire rack to cool completely, or enjoy them slightly warm with a glass of milk.