Step 1: Preheat your oven to 350°F (175°C). Grease and flour an 8 or 9-inch square baking pan, or line it with parchment paper for easy removal.
Step 2: In a small skillet over medium heat, toast the chopped walnuts for 5-7 minutes, stirring frequently, until fragrant and golden. Set aside to cool.
Step 3: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Ensure there are no lumps in the dry mixture.
Step 4: In a large mixing bowl or the bowl of a stand mixer, cream the butter, granulated sugar, and 1/2 cup brown sugar together on medium-high speed until light and fluffy.
Step 5: Add the eggs one at a time, beating well after each addition. Mix in the maple syrup and vanilla extract until fully combined.
Step 6: Reduce the mixer speed to low. Add the dry ingredients in three parts, alternating with the sour cream in two parts, beginning and ending with the flour mixture.
Step 7: Gently fold in 1 cup of the toasted walnuts by hand, being careful not to overmix the batter.
Step 8: Pour the batter into the prepared pan and smooth the top. Bake for 45-55 minutes, or until a toothpick inserted into the center comes out clean.
Step 9: While the cake cools, prepare the glaze. In a small saucepan, melt the 1/4 cup butter and 1/2 cup brown sugar over medium heat. Bring to a gentle boil for 1 minute, then stir in the heavy cream. Remove from heat and whisk in the powdered sugar until smooth.
Step 10: Pour the warm glaze over the slightly cooled cake and sprinkle the remaining 1/2 cup of toasted walnuts on top. Let the glaze set before slicing.