Preheat the oven to 350F. Generously coat a 5 x 9-inch loaf pan with cooking spray. Line the bottom and long sides with a piece of parchment paper, with 1 inch of the paper hanging over the sides of the pan.
Combine the sugar, oil, vanilla, and grapefruit zest in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until the ingredients are incorporated completely and uniform in color. Add the eggs, one at a time, beating to incorporate between additions. Add the baking powder, salt, and baking soda, then beat to incorporate completely. Add the grapefruit juice and beat to incorporate.
Add the flour and blend on low speed until the flour is incorporated. Add the sour cream and beat until incorporated. Scoop one-quarter of the batter into a medium bowl and stir in the tea leaves and cocoa powder until uniform in color. Pour half the plain grapefruit batter into the pan, then half the Earl Grey batter, then the rest of the grapefruit batter and the rest of the Earl Grey batter. Using a butter knife, swirl the batter together a few times, making “figure eight” motions in the batter.
Bake until a skewer or toothpick inserted in the center of the loaf comes out clean, 60 to 70 minutes. Let cool in the pan for about 5 minutes before using the parchment paper overhand to lift the loaf out of the pan and transfer it to a wire rack to cool completely.
While the loaf cools, make the honey grapefruit cream for your cake toast. Swirl honey and 1 teaspoon of grapefruit zest into sour cream, reserving the second teaspoon of zest to garnish. Cover, and place in the fridge while the loaf cools.
Once the loaf has cooled, slice thickly, and toast your slices to desired level of cozy toastiness. Top with dollops of honey grapefruit cream and grapefruit zest, and serve immediately.