Place a rack in middle of oven; preheat to 325°. Generously grease bottom and sides of springform pan with butter. Melt 4 Tbsp. butter in a small saucepan over medium heat until bubbling, about 2 minutes. Add brown sugar and ¼ tsp. Diamond Crystal or Morton kosher salt; cook, stirring constantly, until brown sugar is dissolved and butter is copper colored, about 30 seconds. (It’s okay if mixture doesn’t come together at this point.) Pour caramel into prepared pan.
 
#### Step 2
Carefully arrange bananas, cut side down, in pan, cutting them to fit in an even layer.
Whisk together flour, almond meal, baking powder, and remaining ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a medium bowl. Whisk egg, egg yolk, coconut milk, oil, vanilla, and 1 cup granulated sugar in a large bowl to combine. Add dry ingredients and mix with a rubber spatula until just combined. Pour batter over bananas and gently tap pan on counter to release any air bubbles.
 
#### Step 3
Place pan on a foil-lined rimmed baking sheet and transfer to oven. Bake cake, rotating pan halfway through, until golden brown and a tester inserted into the center comes out clean, 45–60 minutes. Immediately run an offset spatula around edges of pan to loosen cake. Transfer pan to a wire rack and let cake cool in pan 20 minutes; reserve baking sheet.
 
#### Step 4
Invert baking sheet over top of pan, then flip cake over onto baking sheet. Remove ring from pan; carefully lift away pan bottom, running offset spatula between bananas and bottom to release (don’t worry if some of the caramel sticks).
 
#### Step 5
Sprinkle remaining ¼ cup granulated sugar evenly over cake. Using kitchen torch, melt sugar until deep amber. Sprinkle cake with sea salt.