Line cake pan with parchment paper cut to size of pan and grease well.
In a medium-sized mixing bowl, sift or whisk the1 cup all-purpose flour, 2 teaspoons baking powder and 1/4 teaspoon salt.
In a large mixing bowl, cream the 3 Tablespoons unsalted butter, with the 1/2 cup granulated sugar. Add in 1 large egg and 2 teaspoons orange zest.
Add in half of the dry ingredients, followed by the 2 tablespoons milk and 2 tablespoons orange juice and then the other half the dry ingredients last. Only mix until combined.
Spread batter into prepared cake pan. In a small mixing bowl, combine the 1 Tablespoon all-purpose flour with the 2 tablespoons brown sugar,1 Tablespoon granulated sugar ,1/4 teaspoon ground cinnamon and 1 Tablespoon unsalted butter. Mix until crumbly and then sprinkle on top of cake.
Bake at 350°F 20-25 minutes, or a until a toothpick inserted in the center of the cake comes out clean.
Allow cake to cool on wire rack in pan for 15 minutes; then run knife around edges of cake and remove cake from pan. Allow to cool completely on wire rack before removing parchment paper.
Whisk remaining 3 teaspoons orange juice with the 1 cup powdered sugar and remaining 2 teaspoon orange zest. Drizzle over cake; allow to set slightly before serving.