Whisk together the flour, graham cracker crumbs, baking powder, baking soda, salt, and cinnamon. Set aside.

Using an electric mixer on medium speed, beat the butter and brown sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Mix in the vanilla.

Reduce mixer speed to low. Add the flour mixture in three portions, alternating with two portions of the milk. Mix just until combined.

Transfer the batter to the prepared pan. Spread evenly.

Bake 40 to 45 minutes, or until a pick inserted into the center comes out clean.

Cool the cake in the pan on a wire rack for 15 minutes. Then transfer the cake from the pan to a wire rack to cool completely.

Place the butter and chocolate in a microwave-safe bowl. Heat at half-power in 30-second increments until the butter has melted and the chocolate melts when stirred.

Gradually stir in the confectioners' sugar, adding a small amount of water at a time as needed for mixing. (You may not use all of the water.) Stir until smooth.

Working quickly before the glaze sets, drizzle the glaze over the cooled cake. Top with graham cracker crumbs.