Beat the brown sugar, baking powder, cocoa powder, cinnamon, salt, baking soda and 2 cups of flour in a bowl and mix.
Add 1 egg until mixture resembles coarse sand. Add sour cream and vanilla and continue to beat until the batter is thick and smooth. Approximately 1 minute later, add another egg and continue to mix. Repeat with the remaining egg and scrape batter into prepared tin.
Bake in the Russell Hobbs air fryer basket for 18 minutes at 200 degrees until a cake tester inserted into the centre comes out clean and the cake springs back slightly when pressed. Let the cake cool down for at least 20 minutes in the pan, then turn out the cake onto a cooling rack to finish cooling down completely.
Melt brown sugar and butter in a small saucepan over medium-low heat, whisking constantly. Remove pot from heat and whisk in vanilla, sour cream, and sea salt until smooth.
Drizzle ½ cup of sauce over the cake. Sprinkle with more sea salt. Serve cake with sour cream and remaining toffee sauce.
