In a food processor, pulse walnuts to the texture of cookie crumbs. Set aside.
In a large bowl, whisk together dry ingredients; then stir in walnuts. In a separate bowl, whisk together butter and eggs; then drizzle in milk and whisk well. Add wet ingredients to dry ingredients; then stir until just blended. Let batter rest 8–10 minutes.
Heat a lightly buttered griddle or cast-iron skillet over medium-high heat. Ladle in batter. Cook 2 minutes on one side, until bubbles begin to appear on the surface. Flip pancakes and cook 2 minutes longer, or until browned.
Repeat with remaining batter. Serve with maple syrup.