Add the eggs to the mixture one at a time, mixing well after each addition until everything is well combined.
Fold through the sour cream or yoghurt, then add the dry ingredients and continue folding until just combined - be careful not to over mix! Pop the cake mixture aside.
In a small bowl prepare the filling by mixing together the brown sugar and cinnamon. Pop the filling aside.
In a medium bowl, mix together all of the crumble ingredients until well combined.
Pour half of the cake mixture into the prepared baking tin and spread evenly to the edges.
Sprinkle the cinnamon sugar evenly across the top of the cake mixture.
Top with the remaining cake batter and carefully spread to the edges of the cake tin.
Top evenly with the crumble topping, using your hands to make nice clusters.
Bake the cake for 30 - 40 minutes or until the cake is springy to touch and a skewer inserted comes out clean.
Leave to cool before removing from cake tin.
Wahoo, you should now have an irresistibly delicious Smooth Peanut Butter crumble cake! This cake is the perfect balance of sweetness and even those non-coffee drinkers will love it! Store in an airtight container for up to 5 days. Serve with cream or yoghurt.