- For streusel: In a medium bowl, whisk together Heilala Vanilla Sugar, brown sugar, flour, and salt. Add the Heilala Cocoa Vanilla Extract and the melted butter, and stir with a spatula until clumps form. Set aside.
- Preheat the oven to 165°C. Grease a 23cm square baking pan, and line it with parchment paper.
- For cake: In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar together on high speed until light and fluffy, about 3 to 5 minutes. Add the egg and Heilala Cocoa Vanilla Extract and mix to combine.
- In a medium bowl, sift together the flour, baking powder, and salt. With the mixer on low, add about a third of the flour mixture, followed by half of the yogurt and another third of the flour and then the rest of the yogurt, followed by the remainder of the flour. Mix on low until almost combined—you want to see some flour remaining in the bowl.
- Remove the bowl from the mixer and add in the chopped chocolate. Finish mixing the cake batter by hand to finish off the mixing process and incorporate the chocolate. Transfer the cake batter in a few large dollops to the prepared pan, and smooth it down with an offset spatula.
- Sprinkle the prepared streusel evenly over the surface of the cake, breaking up any large clumps with your fingers, and pressing down lightly to help adhere.
- Bake the coffee cake for 50 minutes to an hour, or until a instant read thermometer inserted into the center of cake reads 93°C.
- Allow to cool in the pan before slicing into squares. Store leftovers in an airtight container at room temperature.