FOR THE CAKE:
PREHEAT OVEN TO 350F DEGREES. SPRAY 9 X 13 BAKING DISH WITH NONSTICK COOKING SPRAY (I LIKE THE KIND THAT HAS FLOUR IN IT.) IN A LARGE BOWL (OR THE BOWL OF YOUR STAND MIXER), COMBINE INGREDIENTS FOR CAKE (EXCEPT MELTED BUTTER).
WHILE YOUR MIXER IS RUNNING, SLOWLY ADD THE 1/2 CUP MELTED BUTTER. MIX JUST UNTIL COMBINED (OVERMIXING FLOUR IN BAKED GOODS CAN MAKE IT TOUGH AND RUBBERY.)
SPREAD THIS BATTER EVENLY INTO YOUR BAKING DISH.
FOR THE FILLING:
IN A SMALL BOWL, COMBINE ALL YOUR FILLING INGREDIENTS: SOFTENED BUTTER, BROWN SUGAR, FLOUR AND CINNAMON. STIR UNTIL COMBINED WELL.
DROP THE CINNAMON FILLING BY SPOONFULS ONTO THE CAKE BATTER.
TAKE A BUTTER KNIFE AND MAKE SWIRLY, MARBLE-EFFECT PATTERNS ALL AROUND THE CAKE. DRAG THE KNIFE THROUGH BOTH THE CINNAMON AND CAKE BATTER.
BAKE THE CAKE FOR ABOUT 35-40 MINUTES (IT TOOK ABOUT 40 MINUTES IN MY OVEN), UNTIL A KNIFE INSERTED INTO THE CENTER COMES OUT CLEAN.
FOR THE GLAZE:
WHISK TOGETHER POWDERED SUGAR, MILK AND VANILLA EXTRACT IN SMALL BOWL.
POUR THE GLAZE OVER THE WARM CAKE.