For the lemon butter: Add the butter, cardamom, lemon zest and salt to a bowl. Mix until combined. Turn out onto a piece of plastic wrap and roll into a log. Refrigerate until it hardens, at least 2 hours.
For the waffles: Combine the flour, cardamom, baking powder, salt and baking soda in a bowl and whisk together. Mix the milk, yogurt, grapeseed oil and egg yolks together in another bowl. Whisk the egg whites with the sugar in another bowl until nearly stiff peaks form.
Combine the flour mixture with the milk and yogurt mixture, then fold in the egg whites.
Preheat the oven to 200 degrees F. Preheat a waffle iron and line a baking tray with a wire rack.
Grease the waffle iron with a little butter. Cook waffles according to the waffle iron instructions. Remove to the prepared baking tray. Place in the oven until all the waffles are made and ready to eat.
For serving: Stack 2 waffles and top with a 1/2-inch-thick piece of lemon butter. Sprinkle with ground pistachios, top with seasonal fruit and drizzle with maple syrup. Garnish with fresh mint and serve.