For the Crumb Topping:
30g White sugar
30g Brown sugar
Pinch of Salt
55g Butter, melted and still warm
100g Cake flour (I used Lighthouse cake flour)
For the Cake:
140g Cake flour
¼ teaspoon Bicarb soda
¼ teaspoon Salt
85g Butter, cut into 6 pieces, softened but still cool
1 Egg yolk
1 teaspoon Vanilla extract
85g mini chocolate chips (I used both white and milk)
80g Mini chocolate chips
Icing sugar, to dust
To Make the Crumb Topping, In a medium bowl, whisk the sugars, salt and butter to combine. Add the flour and stir with a wooden spoon until the mixture is a thick dough; set aside to cool to room temperature, 10 to 15 minutes.
To make the Cake, Preheat oven to 160'. Spray an loaf pan with nonstick cooking spray and line with baking paper; allow excess to overhang edges of pan (This helps lift the cake out at the end).
Using an kitchen-aid mixer (with paddle attachment) on low speed, mix the flour, sugar, bicarb soda and salt to combine. With the mixer continuing to run at low speed, add the butter one piece at a time; continue beating until the mixture resembles breadcrumbs. Add the egg, yolk, vanilla, and buttermilk; beat on medium-high speed until light and fluffy, about 1 minute, scraping if necessary. Using a spatula, fold the chocolate chips into the batter. Transfer the batter to the loaf pan and spread into even layer. Break apart the crumb topping into large pea-sized pieces and spread in even layer over the batter. Bake until the crumbs are golden and a wooden skewer inserted into center of cake comes out clean, 45 to 55 minutes. Immediately after taking the cake out of the oven, sprinkle with mini chocolate chips. Cool on a wire rack at least 30 minutes. Remove the cake from the pan. Dust with icing sugar before serving.