Step 1

Place a rack in center of oven; preheat to 350°. Generously grease an 8" or 9" round pan with butter. Line bottom with parchment and grease parchment with butter.

Step 2

Toast black sesame seeds in a dry large skillet over medium heat, stirring frequently, until fragrant and just starting to crackle, 2–3 minutes. Transfer to a food processor or blender. Pulse, scraping down sides as needed, until seeds are finely chopped but haven't formed a paste. This could take as long as 2–3 minutes in a food processor, or 1–2 minutes in a high-speed blender.

Step 3

Scoop out ¼ cup ground sesame seeds; set aside. Add ¾ cup sugar to food processor and pulse until sugar is a uniform gray and no lumps of sesame remain. Transfer to a large bowl. Sprinkle some sesame sugar all over sides and bottom of prepared pan (as if you were flouring it). Tap pan so sesame sugar coats sides and bottom in a thin layer; this will form a crust, which will help the cake release. Tap out any excess back into bowl. Add flour, baking powder, and ½ tsp. salt to bowl and whisk to combine.

Step 4

Whisk eggs and ¼ cup melted butter in a medium bowl until creamy and emulsified, about 30 seconds. Whisk in milk, vanilla, and ½ cup cream. Add egg mixture to dry ingredients and stir to combine.

Step 5

Transfer batter to prepared pan. Bake cake until a tester inserted into the center comes out clean and cake is risen and springy with a firm golden brown crust, 45–55 minutes (the smaller your pan, the longer it will take).

Step 6

Run a knife or offset spatula around edge of pan. Let cake cool in pan 10 minutes before inverting onto a wire rack. Invert again onto another wire rack and let cool completely.

Step 7

Vigorously whisk reserved ¼ cup ground sesame seeds with remaining 5 Tbsp. cream and ½ tsp. salt in a medium bowl until smooth.

Step 8

Bring cream of tartar, remaining ½ cup sugar, and 3 Tbsp. water to a boil in large skillet over medium-high heat, stirring to dissolve sugar. Cook, stirring frequently, until mixture is the color of light honey, 3–4 minutes.

Step 9

Immediately remove caramel from heat. Vigorously whisk in cream mixture until combined. Immediately pour onto center of cooled cake and spread to the edges. Garnish circumference of cake with white sesame seeds.