Prepare the pan: Lightly grease your 6- to 7-inch pan. Line the bottom with parchment if you’d like cleaner slices. Set aside.

Preheat the air fryer: Set to 320°F (160°C) for 5 minutes. Preheating helps the cake rise evenly and avoids a gummy center.

Make the streusel: In a small bowl, combine flour, brown sugar, cinnamon, and salt. Add cold butter cubes and cut in with a fork until you have pea-sized crumbles. Stir in nuts if using. Refrigerate while you mix the batter to keep it crumbly.

Mix dry ingredients: In a medium bowl, whisk flour, baking powder, baking soda, and salt. This keeps the crumb light and uniform.

Cream butter and sugar: In a separate bowl, beat the softened butter and granulated sugar until fluffy and pale, about 1–2 minutes. Add the egg and vanilla, and mix until smooth.

Add sour cream and milk: Blend in the sour cream until fully incorporated, then mix in the milk. The batter will look silky and slightly thick.

Combine: Add the dry ingredients to the wet and fold gently with a spatula until just combined. Avoid overmixing to keep the cake tender.

Layer the batter and streusel: Spread half the batter into the prepared pan. Sprinkle about one-third of the streusel over it. Add the remaining batter and smooth the top. Finish with the remaining streusel in an even layer.

Air fry: Place the pan in the air fryer basket. Cook at 320°F (160°C) for 22–28 minutes, depending on your model and pan size. The cake is done when a toothpick inserted in the center comes out mostly clean with a few moist crumbs.

Rest and cool: Let the cake sit in the pan for 10 minutes. Run a knife around the edge, then lift out or release the springform. Cool another 10–15 minutes before slicing to keep the crumb from crumbling.

Optional glaze: Stir 1/2 cup powdered sugar with 1–2 teaspoons milk and a drop of vanilla to make a drizzle. Spoon over the cooled cake for extra sweetness.