Cinnamon Streusel Loaf Cake

Preheat your oven to 350°F and grease a 9x5-inch loaf pan thoroughly.

In a large bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon until evenly distributed. (Use Ceylon cinnamon if you can find it - it's sweeter and more delicate than cassia cinnamon, perfect for this tender cake.)

For the streusel, use your fingertips to cut the cold butter into the dark brown sugar and granulated sugar until you have a wet, crumbly mixture with some larger butter pieces.

In another bowl, whisk together the melted butter, light brown sugar, and granulated sugar until smooth and well combined.

Add the egg whites to the sugar mixture and beat vigorously for 2-3 minutes until the mixture is pale and slightly thickened.

Stir in the buttermilk until just combined.

Add the wet ingredients to the dry ingredients in two additions, stirring just until the flour disappears. Don't overmix.

Pour the batter into your prepared loaf pan and smooth the top gently.

Sprinkle half of the streusel mixture over the batter, then use a knife to cut through and create marbled swirls.

Top with the remaining streusel mixture.

Bake for 60-75 minutes, until a knife inserted in the center comes out with just a few moist crumbs. (The caramelized top will be deeply golden and may crack beautifully - this is what you want.)

Cool in the pan for 10 minutes before turning out onto a wire rack.