Adjust an oven rack to the middle position and heat the oven to 350 degrees. Grease an 8 1/2 by 4 1/2 inch loaf pan with 1 tablespoon of the softened butter; dust with 1 tablespoon of the flour, tapping our the excess. In a medium bowl, whisk together the remaining 1 1/2 cups flour, the baking powder and the salt; set aside.
Melt the remaining 16 tablespoons butter in a small saucepan over medium heat. Whisk the melted butter thoroughly to reincorporate any separated milk solids.
In a food processor, pulse the sugar and zest until combined, about 5 pulses. Add the lemon juice, eggs and vanilla; process until combined, about 5 seconds. With the machine running, add the melted butter through the feed tube in a steady stream [this should take about 20 seconds]. Transfer the mixture to a large bowl. Sift the flour mixture over the egg mixture in three additions, whisking gently after each addition until just combined.
Pour the batter into the prepared pan and bake 15 minutes. Reduce the oven temperature to 325 degrees and continue to bake until deep golden brown and a toothpick inserted in the center comes out clean, about 35 minutes, rotating the pan halfway through the baking time. Cool in the pan for 10 minutes, then turn onto a wire rack. Pike the top and sides of the cake throughout with a toothpick. Cool to room temperature, at least 1 hour. [The cooled cake can be wrapped tightly in plastic wrap and stored at room temperature for 5 days.].
For the glaze: While the cake is cooling, bring the sugar and lemon juice to a boil in a small saucepan, stirring occasionally to dissolve the sugar. Reduce the heat to low and simmer until thickened slightly, about 2 minutes. Brush the top and sides of the cake with the glaze and cool to room temperature.