Place the salted butter in a small saucepan over medium heat. As the butter melts, it will begin to foam. Continue cooking, stirring or swirling the pan frequently.

The butter will gradually turn golden brown and develop little brown flecks along the bottom of the pan. Once it smells nutty and the milk solids are a deep golden brown, immediately remove it from the heat.

Pour the browned butter, including all of the brown bits, into a heatproof bowl. Allow it to cool until warm but no longer hot before using.

In a small bowl, combine the melted butter, brown sugar and flour. Mix with a fork until the mixture forms coarse, crumbly pieces. Set aside.

Chop the apples and mix them with the 2 teaspoons of flour. Set them aside while you preare the rest of the batter.

In a large mixing bowl, whisk together the cooled browned butter and granulated sugar.

Add the eggs and whisk until well combined.

Add the Greek yogurt, sourdough discard and vanilla extract. Whisk until smooth and fully incorporated.

Add the dry ingredients to the wet ingredients. Stir gently with a spatula until just combined. Be careful not to overmix.

Fold in the flour-coated chopped apples until evenly distributed throughout the batter.

Spread the batter evenly into the prepared pan.

Sprinkle the brown sugar streusel evenly over the top of the batter.

Bake at 350°F (175°C) for approximately 50–55 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs.

Allow the coffee cake to cool in the pan for about 15–20 minutes before carefully removing the sides of the springform pan.

In a small bowl, whisk together the powdered sugar, milk and vanilla extract until smooth.

If the icing is too thick, add a tiny splash of milk. If it is too thin, add a little more powdered sugar.

Drizzle the vanilla icing over the cooled coffee cake before serving.