On 350 degrees adjust the cake rack in the middle section of your oven. Grease an 8 1/2 by 4 1/2 cake pan with softened butter. Dust with all purpose flour and tap out excess.
In a medium bowl add 1 1/2 cups flour, baking powder, and salt. Set aside.
In medium pot add 1/2 cup butter melt with 4 stalks lemongrass chopped and crushed on low heat, let steep for 1 hour, strain through fine china cap. be sure to extract as much lemongrass flavor as possible.
In food processor pulse sugar and zest about 5 pulses should do it. Add lemon juice, eggs, and vanilla, process 5 seconds. While running slowly add remaining melted butter, no longer than 20 seconds.
Transfer mix to large bowl and sift the rest of the flour in 3 additions whisking gently after each one.
Pour mixture into pan and bake for 15 minutes then reducing heat to 325 degrees and bake for another 35 minutes rotating half way through.
When done test with tooth pick, and let rest in pan for 10 minutes. then transfer to wire rack and let cool for at least another hour.