Preheat the oven to 350°F. Butter and flour a standard 9 inch loaf pan or spray with baking spray.
In a small bowl, sift together the flour, sugar and baking powder. Set aside.
Peel and finely chop the orange pulp removing any seeds and pith, to equal 2/3 cup.
In a medium size bowl, whisk together orange pulp, butter, eggs, buttermilk, lemon zest and vanilla until fully blended.
Add the sifted dry ingredients. Use a large spoon and mix by hand until fully combined. Spread the batter into the loaf pan.
Bake for 36-38 minutes or until golden and a toothpick inserted into the center comes out clean. Cool in the pan for 15 minutes, then remove carefully to cool completely on a cooling rack.
To make the citrus drizzle: Mix together the powdered sugar, heavy cream and orange zest. Stir until blended. Add the juice, as needed to thin to drizzle consistency. Drizzle over cooled loaf.
Garnish with strips of orange zest, if desired.