Prep the pan and air fryer: Line the bottom of a 6- or 7-inch round cake pan with parchment and lightly grease the sides. Preheat the air fryer to 320°F (160°C) for 3–5 minutes.

Make the streusel: In a small bowl, mix flour, brown sugar, cinnamon, and salt. Cut in the cold butter with a fork until crumbly with pea-sized bits. Stir in nuts if using. Chill while you make the batter.

Mix dry ingredients: In a medium bowl, whisk flour, baking powder, baking soda, and salt.

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

Combine batter: Add half the dry ingredients to the butter mixture, then the sour cream, and finish with remaining dry ingredients. Add milk 1 tablespoon at a time until the batter is thick but spreadable.

Layer the cake: Spread half the batter into the prepared pan. Mix the brown sugar and cinnamon for the swirl, then sprinkle it evenly over the batter. Dollop the remaining batter on top and gently spread to cover.

Add streusel: Sprinkle the chilled streusel over the batter, pressing lightly so it adheres.

Air fry: Place the pan in the air fryer basket. Cook at 320°F (160°C) for 20–25 minutes, until the center springs back and a toothpick comes out clean or with a few moist crumbs. If the top browns too fast, tent loosely with foil during the last 5–7 minutes.

Cool and glaze: Let the cake cool in the pan for 10 minutes, then lift out using the parchment. Whisk powdered sugar, milk, and vanilla for the glaze and drizzle over the warm cake if you like.

Serve: Slice and serve warm or at room temperature with coffee or tea.