Preheat the Oven: Set the oven to 350°F (175°C). Line a 9x9-inch pan with parchment paper.
Make the Crust: Combine flour and powdered sugar in a bowl. Cut in cold butter until crumbly.
Press the Crust: Press the mixture into the prepared pan for an even base.
Bake the Crust: Bake for 15 minutes until lightly golden.
Prepare the Filling: Whisk eggs, sugar, flour, lemon zest, lemon juice, and baking powder until smooth.
Pour the Filling: Pour the lemon mixture over the baked crust.
Bake the Bars: Bake for 18-20 minutes until the filling is set and golden around the edges.
Cool the Bars: Let the bars cool completely on a wire rack.
Make the Glaze: Whisk powdered sugar, espresso, and vanilla until smooth.
Drizzle the Glaze: Drizzle the espresso glaze over the cooled lemon bars.
Slice and Serve: Slice into squares and serve!