Preheat the oven to 180C, fan-forced. Grease a 20cm x 30cm slice pan and line the base and long sides with baking paper, allowing the paper to extend over the sides.
Step 2 — Make cake batter
Put the vanilla cake mix, 180ml (¾ cup) milk, 60ml (¼ cup) vegetable oil and 1 egg in a large bowl and whisk for 1 min or until well combined.
Step 3 — Make streusel topping
Combine the cinnamon, half the brown sugar* and 1½ tbs plain flour in a large bowl. Coarsely chop 25g butter and add to the sugar mixture. Using your fingertips, rub in the butter until it resembles sandy crumbs.
Step 4 — Bake cake
Pour half the batter into the pan, then scatter over half the streusel topping. Pour over the remaining batter and scatter with the remaining topping. Bake for 30 mins or until golden and a skewer inserted into the centre comes out with a few moist crumbs clinging. Cool in the pan for 15 mins, then transfer to a wire rack to cool completely.
Step 5 — Serve up
Dust the streusel cake with icing sugar*, then cut into pieces and enjoy! Serves 8.
Step 6 — Make it yours
Serve with whipped cream or a lemon cream cheese drizzle for extra yum factor!