Line two baking sheets with parchment paper. Put the hazelnuts in a small skillet over medium heat and toast, tossing, until light golden brown and toasty-smelling, about 3 minutes. Transfer to a cutting board and let cool slightly. Chop finely.
Fill two small saucepans with a few inches of water and bring to a bare simmer over low heat. Put the white chocolate and the milk chocolate in separate bowls just large enough to fit snugly over the saucepans without touching the water. Set the bowls over the simmering water and cook, stirring occasionally, until the chocolates are almost completely melted. Remove the bowls from saucepans. Stir the vanilla bean seeds into the white chocolate, and the espresso powder into the milk chocolate; stir each until completely melted and smooth.
Dip the bottoms of the blackberries first in the white chocolate and then in the chopped hazelnuts. Place on one of the lined baking sheets. Dip the strawberries in the semisweet chocolate, shaking off any excess. Place them on the second baking sheet. Refrigerate both baking sheets until the chocolate is set, about 30 minutes. Serve on a small platter.
Recipe courtesy of Bobby Flay