Line a baking sheet with parchment or waxed paper.

Put the bittersweet and semisweet chocolate chips and 1 tablespoon of the butter in a stainless steel bowl set over a pot of simmering water over medium heat. Stirring, melt the chocolate, about 5 minutes.

Melt the white chocolate the same way as the dark chocolates without using any butter.

Heat the remaining tablespoon butter in a small skillet over medium-high heat. Add the nuts and shake the pan often to lightly toast them, 3 to 4 minutes. Add the nuts to the chocolate and cook, stirring the mixture often, until the chocolate is completely melted and the nuts are well coated, about 5 minutes.

Pour the chocolate mixture into the prepared pan and spread it evenly with a rubber spatula over the bottom, making a one-fourth-inch layer. Drizzle the white chocolate over the dark chocolate and using a knife swirl the white chocolate into the dark creating a marbling effect.

Cool. Refrigerate until it sets, about 2 hours. Break it into pieces, like peanut brittle. Store in an airtight container in the refrigerator. It will keep up to 2 weeks.