• Preheat and Prep: Preheat the oven to 360°F (180°C). Generously grease an 8×8-inch metal baking pan with butter, then dust lightly with flour.
  butter and flour

Make the "Buttermilk":

• Stir the lemon juice into the room temperature milk. Set it aside for 5–10 minutes to allow it to thicken and curdle slightly. This creates our DIY buttermilk replacement.
  2 teaspoons (10 ml) lemon juice or white vinegar, 2/3 cup whole milk

Prepare the Cinnamon Filling:

• In a small bowl, whisk all the cinnamon filling ingredients together (granulated sugar, brown sugar, flour, and cinnamon) until combined. Set aside.
  3 Tablespoons granulated sugar, 3 Tablespoons brown sugar, 2 Tablespoons all-purpose flour, 2 teaspoons ground cinnamon

Prepare the Streusel Topping:

• In a separate small bowl, toss the Streusel Topping ingredients together with the melted butter. Stir with a fork until the mixture forms coarse, buttery crumbs. Set aside.
  1/2 cup granulated sugar, 1/2 cup all-purpose flour, 2 teaspoons ground cinnamon, 1/4 teaspoon salt, 3 Tablespoons 45g unsalted butter

Make the Cake

• In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the softened butter and granulated sugar together on medium-high speed. Beat for a full 4–5 minutes until the mixture is pale, light, and very fluffy. Add the vanilla extract and beat on medium until combined.
  1 stick (115g) unsalted butter, softened to room temperature, 1 cup (200g) granulated sugar, 1/2 teaspoon pure vanilla extract
• Add the eggs one at a time, beating on medium speed until just incorporated after each addition. If the mixture looks slightly "curdled" at this stage, don't worry—that's normal! Scrape down the sides and bottom of the bowl with a rubber spatula to ensure everything is even. Set aside
  2 large eggs
• In a medium bowl, whisk the flour, baking powder, baking soda, and salt together until thoroughly combined.
  1⅔ cups (200g) all-purpose flour (spooned & leveled), 1 and 1/4 teaspoons baking powder, 1/4 teaspoon baking soda, 1/2 teaspoon salt
• With the mixer on low speed, add the dry ingredients in three additions, alternating with the milk mixture, beginning and ending with the dry ingredients. Mix until just combined. The batter will be very thick and creamy; avoid overmixing, which can lead to a tough cake.
• Spoon about half of the thick batter into the prepared pan and spread it into an even layer. Evenly sprinkle the Cinnamon Filling over the batter. Carefully dollop the remaining batter over the filling and spread it out as best you can. Finally, sprinkle the Streusel Topping generously over the top.
• Bake for 40–45 minutes. The cake is done when a toothpick inserted into the center comes out clean or with just a few moist crumbs.
• Allow the cake to cool in the pan on a wire rack for at least 20 minutes before slicing. This allows the crumb to set and makes it much easier to cut into clean slices.