Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang for easy removal.

Make the crumb dough:

In a large bowl, whisk together flour, sugar, baking powder, salt, and lemon zest.

Add cold butter and use a pastry cutter or your fingertips to work it into the flour until the mixture resembles coarse crumbs.

In a small bowl, beat the egg with vanilla extract and mix it into the crumb mixture. Stir until the dough begins to come together but still looks crumbly.

Assemble the base:

Press about two-thirds of the crumb dough into the prepared pan to form a firm, even base.

Make the lemon filling:

In a medium bowl, whisk together sweetened condensed milk, lemon juice, lemon zest, and egg yolks until smooth and thick.

Layer the filling:

Pour the lemon filling over the crust and spread evenly with a spatula.

Sprinkle the remaining crumb dough over the top of the filling.

Bake:

Bake for 30–35 minutes or until the top is lightly golden and the filling is just set.

Cool and chill:

Let the bars cool completely in the pan. Then refrigerate for at least 1–2 hours before slicing into squares.

Serve:

Slice into 16–20 bars and dust with powdered sugar before serving if desired.