16 Tbsp unsalted butter, plus 1 Tbsp softened, for greasing pan
1 ½ cups cake flour, plus 1 Tbsp for dusting pan
1 tsp baking powder
½ tsp salt
1 ¼ cups sugar
2 Tbsp grated zest plus 2 Tbsp juice from 2 medium lemons
4 large eggs
1 ½ tsp vanilla extract
½ cup sugar
¼ cup juice from 1 or 2 medium lemons
Adjust oven rack to middle position and heat oven to 350 F. Grease 9x5-inch loaf pan with 1 Tbsp softened butter; dust with 1 Tbsp cake flour, tapping out excess. In medium bowl, whisk together flour, baking powder, and salt; set aside. In glass measuring cup or microwave-safe bowl, microwave butter, covered with plastic wrap, at full power until melted, 1 to 2 minutes. (Alternatively, melt butter in small saucepan over medium heat.) Whisk melted butter thoroughly to reincorporate any separated milk solids. In food processor, process sugar and zest until combined, about five 1-second pulses. Add lemon juice, eggs, and vanilla; process until combined, about 5 seconds. With machine running, add melted butter through feed tube in steady stream (this should take about 20 seconds). Transfer mixture to large bowl. Sift flour mixture over eggs in three steps, whisking gently after each addition until just combined.
Pour batter into prepared pan and bake 15 minutes. Reduce oven temperature to 325 F. and continue to bake until deep golden brown and skewer inserted in center comes out clean, about 35 minutes, rotating pan halfway through baking time. Cool in pan for 10 minutes, then turn onto wire rack and brush on Lemon Glaze, if desired. Cool to room temperature, at least 1 hour.
Brush this glaze onto the warm cake to give it a fresh sweet-tart lemon kick. While cake is cooling, bring sugar and lemon juice to boil in small non-reactive saucepan, stirring occasionally to dissolve sugar. Reduce heat to low and simmer until thickened slightly, about 2 minutes. Apply glaze.