Preheat oven to 350°F (175°C). Grease and fully line a 9x5-inch loaf pan with parchment paper.

Make the streusel. In a medium bowl, combine the flour, granulated sugar, brown sugar, sanding sugar, and salt. Add the cold butter, peanut butter, and vanilla bean paste. Work together until crumbly with a mix of small crumbs and larger clusters. Fold in the mini chocolate chips and chopped peanut butter cups. Refrigerate while preparing the batter.

In a large bowl, whisk together the granulated sugar, brown sugar, oil, yogurt, milk, vanilla bean paste, and apple cider vinegar until smooth.

Add the flour, cornstarch, baking powder, baking soda, salt, and nutmeg. Whisk just until combined and no dry streaks remain.

Fold in the chopped chocolate just until evenly distributed.

Transfer the batter to the prepared loaf pan and smooth the top. The batter should fill the pan just over halfway full.

Sprinkle the chilled streusel evenly over the batter, leaving some larger clusters intact.

Sprinkle the top lightly and evenly with coarse sanding sugar just before baking.

Bake for 45 minutes, or until the dome is set, the streusel is crisp, and a toothpick inserted into the center comes out with moist crumbs and no wet batter.

Cool in the pan for 10 minutes, then sprinkle the chopped peanut butter cups over the top, followed by the Maldon flaky sea salt.

Cool in the pan for 5 more minutes, for 15 minutes total, then transfer the loaf to a wire rack.

Allow the loaf to cool completely at room temperature.

Chill for about 30 minutes, or until the center and topping feel set enough for clean slices. If you do not want to wait on the chill, let it set on the counter and serve it warmer.