Place 1 cup sugar in an 11" skillet over medium heat. Shake skillet to evenly distribute sugar over bottom of skillet. When sugar begins to melt (clear liquid will form around edges), shake pan as needed to keep sugar evenly covering the bottom of the skillet. When sugar in center of pan begins to turn golden (this will take about 6 minutes), stir with a wooden spoon just often enough to maintain an even color, shaking pan and allowing sugar to continue to melt between stirrings. When sugar is completely melted and amber in color, quickly spoon about 1 Tbsp. syrup into each of 6 (6-oz.) custard cups, tilting to coat bottoms evenly. Set cups in a 13" x 9" pan.
Mash bananas slightly with a fork to measure 3/4 cup. Place banana in blender; add remaining 1/2 cup sugar, vanilla bean paste, and half-and-half. Process 5 seconds or just until smooth. Add eggs; process 5 seconds or just until thoroughly blended, but not foamy. Be sure not to overprocess the eggs. Pour custard evenly into prepared cups. Add hot water to pan to depth of 1".
Bake, uncovered, at 325° for 40 minutes or until custards are set and a knife inserted near center comes out clean.
Remove cups from water bath, and cool completely on a wire rack. Cover and chill at least 8 hours. Run a paring knife around edge of each cup to loosen custards, and invert onto a serving platter, letting melted caramel drizzle down sides of custards. Top each dessert with a walnut half.
Tip: It's easiest if you place the pan containing filled custard cups on oven rack before adding hot water. Wear rubber gloves to easily remove cups from water after baking.