Preheat the oven to 180ºC (350ºF) and grease and line a 450g (1 lb) loaf tin with non-stick parchment paper, leaving a couple inches of paper overhanging for easy removal. Set aside.
Make the streusel by whisking together the flour, coconut sugar, cinnamon, and salt in a small bowl. Drizzle over the melted butter and bring it together gently with a fork. All the dry ingredients should be coated in the butter and there should be some smaller and some larger clumps. Refrigerate until ready to assemble the cake.
Make the cake by tipping the flour, coconut sugar, baking powder, and baking soda into a large bowl. Whisk together to combine. Rub the cold butter into the flour mixture with your hands until it’s well-dispersed and the texture resembles sand. Set aside.
In a small bowl, whisk together the sour cream, vanilla, egg, and milk. Pour the wet ingredients into the dry ingredients and mix together until smooth.
Pour half of the cake batter into the prepared loaf tin and level the top, making sure to get the batter to the edges of the pan. Evenly sprinkle over ½ of the streusel mixture. Repeat with the remaining batter and streusel.
Bake the cake for 27–31 minutes, or until a toothpick inserted at the centre comes out clean. Place the tin on a wire rack and allow the cake to cool for 10 minutes, then carefully lift the cake from the tin using the paper tabs and set it on the wire rack to finish cooling completely. Slice into 6 triangles and serve with coffee or tea.