For the Cake:
1½ cups all purpose flour
1 Tbsp instant espresso powder (optional, see note)
1½ tsp baking powder
½ tsp salt
½ cup (1 stick) unsalted butter, softened
1 cup packed light brown sugar
1 egg
1 egg yolk
1½ tsp vanilla extract
1 cup milk
For the Streusel:

4 Heath bars (1.4 oz. each) chopped, or 1 cup prechopped Heath bits
2 tbsp light brown sugar
2 Tbsp all purpose flour
1 Tbsp butter, softened