Makes: 8-inch (20 cm) square cake, cuts into 16 squares
½ cup (125 g) unsalted butter, room temperature
¾ cup (150 g) sugar
1 large egg
1 large egg yolk
½ cup (125 g) sour cream
1 tsp (5 mL) vanilla extract
1⅓ cups (175 g) all-purpose flour
1 tsp (5 mL) baking powder
¼ tsp (1 mL) baking soda
¼ tsp (1 mL) salt
1. Preheat the oven to 350 °F (180 °C). Grease an 8-inch (20 cm) square pan and line the bottom and sides with parchment paper.
2. For the streusel, stir the flour, brown sugar, cinnamon and salt in a small bowl with a fork to combine. Add the melted butter and stir until the mixture is evenly moistened.
3. For the cake, beat the butter and sugar until it is light and fluffy. Add the egg, then the egg yolk, beat well after each addition. Stir in the sour cream and vanilla. In a separate bowl, sift the flour, baking powder, baking soda and salt. Add the flour mixture to the butter mixture and stir just until blended.
4. Spread half of the batter into the prepared pan and sprinkle half of the streusel topping overtop. Spread the remaining batter in the pan and top with the remaining streusel. Bake the cake for 30-40 minutes, until a tester inserted into the centre of the cake comes out clean. Allow the cake to cool completely in the pan, and then cut into squares as needed to serve.
The coffee cake will keep, well wrapped and unrefrigerated, for 3 days.