In a saucepan, bring milk, heavy cream and lemongrass to a simmer. Remove from heat and let steep, covered for about 30 minutes Strain cream mixture through a sieve into a bowl.
Generously butter a bundt pan and dust with flour, knocking out excess flour.
Sift together cake flour and baking powder. Repeat sifting into another bowl. Flour will have been sifted 3 times total.
Beat together butter and sugar at medium-high speed until pale and fluffy.
Add eggs 1 at a time, beating well after each addition.
Reduce speed to low and add half of the flour, then all of the cream, then remaining flour, mixing well after each addition.
Scrape down side of bowl, then beat at medium-high speed for about 2 minutes Batter will become creamier and satiny.
Pour batter into buttered and dusted pan and hit pan against work surface once or twice lightly to eliminate any air bubbles.
Bake at a 160°C pre-heated oven until golden brown and a wooden pick or a skewer inserted in the middle of the cake (about 20-25 mins).