• Preheat oven to 350℉. Butter and sugar standard one pound loaf pan (9×5 or 9X4-inch). Tap out excess sugar. Set aside.
• Whisk together flour, baking powder, and salt in medium bowl. Set aside.
• In the bowl of a stand mixer, rub calamansi zest (if using) and granulated sugar together to extract flavor. Add in softened butter. Secure paddle attachment to stand mixer. Beat together until well combined.
• With the mixer running on medium speed, add eggs one at a time.
Ensuring first egg is incorporated before adding the second one.
• Add half of flour mixture. Briefly mix on low speed. Add in calamansi juice and sour cream. Beat until well combined. Add remaining flour and mix on low speed until there are no longer any dry streaks of flour. Scrape down sides and bottom of bowl as needed to ensuring thorough mixing.
• Transfer batter to prepared loaf pan. Spread into an even layer. Bake for 50-60 minutes until cake springs back to touch and toothpick inserted in center of cake comes out clean.
• Cool in pan for 5-10 minutes and prepare glaze. Run mini offset spatula along the edges of the pan. Invert cake or pull cake out of loaf pan.
You will be add glaze to the cake while it is hot/warm!

Glaze:

• Mix together the hot melted butter and powdered sugar until a smooth paste forms. Add the calamansi juice a little at a time, stirring until fully incorporated. Season with a pinch of salt.
• Using a pastry brush, apply the glaze over the warm cake in stages, making sure to coat the edges.
Allow the cake to absorb each layer before adding more. You'll use all of the glaze.
• Let the cake cool completely to room temperature. As it cools, the glaze will set into a delicate, lightly crisp crust. Slice with a serrated knife and serve.