Step 1: Preheat your oven to 350°F (175°C) and grease a 9-inch bundt pan or deep cake pan thoroughly to ensure your maple walnut cake with brown sugar glaze releases easily.
Step 2: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
Step 3: In a large bowl, cream the butter, granulated sugar, and light brown sugar together until light and fluffy, about 5 minutes.
Step 4: Beat in the eggs one at a time, followed by the vanilla extract and the pure maple syrup, mixing well after each addition to build the base of your maple walnut cake with brown sugar glaze.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the sour cream. Begin and end with the flour mixture.
Step 6: Gently fold in the toasted walnuts using a spatula until just combined.
Step 7: Pour the batter into the prepared pan and bake for 45-55 minutes, or until a toothpick inserted into the center comes out clean.
Step 8: While the cake cools, prepare the glaze by melting the 1/2 cup butter and dark brown sugar in a saucepan over medium heat until bubbling.
Step 9: Remove from heat, whisk in the heavy cream and powdered sugar until smooth.
Step 10: Allow the maple walnut cake with brown sugar glaze to cool in the pan for 10 minutes before inverting onto a wire rack and drizzling with the warm glaze.