Preheat the oven to 180°C (170°C fan). Grease the baking tin and line it with baking paper, leaving plenty of overhang on longer sides. Set aside.
For the streusel topping:
Melt the butter in a small saucepan over a medium heat. In a medium bowl, mix together the flour, sugar and salt. Pour the melted butter over the mixture and using fork, mix to combine, until you have a mixture that resembles the texture of the wet sand and there is a selection of small and big crumbs and clumps. Place the bowl in the freezer, while you prepare the cake.
For the pumpkin filling:
Place all of the ingredients in a small bowl and mix to combine. Set aside until needed.
For the cake:
In a bowl of an electric mixer, cream the butter and sugar together until pale and fluffy, about 3-4 minutes. Add eggs, one at a time, beating well after each addition. Add vanilla and mix to combine. In a medium bowl, mix together the dry ingredients: the flour, baking powder and salt. With the mixer running on low speed, add the flour in 3 additions, alternating with sour cream. Make sure you begin and end with the flour. You should have thick, fluffy and smooth batter at the end.
Transfer roughly about ⅔ of the cake batter into the prepared baking tin and spread an even layer using spatula or the back of the spoon. Next, add spoonful's of pumpkin filling on top of the cake batter, and spread it gently on top. Spoon out the remaining cake batter on top, and gently spread it, trying to cover all of the pumpkin filling. Don't worry too much if few gaps are remaining, just try to cover as much of the filling as possible.
Remove the streusel topping from the freezer and sprinkle it all over the to of the cake in an even layer. If you have any really big clumps/ clusters of streusel, try to break them with your fingers. Transfer the cake into the preheated oven and bake for 45-55 minutes, or until toothpick inserted in the middle comes out clean.
Remove from the oven and allow the cake to cool for about 20-30 minutes, before lifting it from the tin using the overhang baking paper. Slice the cake into 12-16 pieces and enjoy!
If no sour cream is available, use the same amount of full-fat Greek yoghurt.
Store the cake in the airtight container at room temperature for up to 4 days. It will stay moist for days, but the streusel topping will loose it's crunch with time and will become softer.
To freeze, wrap individual slices of cake in a double layer of clingfilm or reusable kitchen wrap, and place in the freezer friendly bag. Freeze for up to 3 months. Thaw at room temperature for 2-3 hours.
To make cinnamon glaze (optional), combine 100g of icing sugar with ½ teaspoon of ground cinnamon and about 1 tablespoon of milk and mix until you have runny consistency, suitable for drizzling.