Preheat the oven to 375 degrees F. Spray a 9-by-5-inch loaf pan with cooking spray.
Whisk together the flour, baking powder, baking soda and salt in a bowl; set aside.
Thoroughly mash the avocado with the lemon juice in a large bowl. Add the sugar, half-and-half, vanilla and egg and whisk until thoroughly combined. Add the flour mixture and stir until incorporated.
Pour the batter into the prepared loaf pan and bake until golden brown on top and a toothpick inserted in the center comes out clean, 45 to 55 minutes. Let cool in the pan for 10 minutes, then remove to a wire rack to cool completely.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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