Make the bars:

1. Position a rack in the middle of the oven and preheat the oven to 325°F (163°C). Butter an 8×8 inch baking pan (preferably light-colored to avoid over-browning) and line with parchment paper. 
2. Make the brown butter. Heat a 10-12-inch skillet over medium heat, and add the butter. Cook, stirring continuously, until the butter foams, stops sizzling, and turns a dark amber brown, about 7 minutes. Pour the butter, including the brown bits at the bottom of the pan, into a clean bowl or glass measuring cup and set aside until it is cool enough to touch. 
3. Meanwhile, in a medium bowl, whisk together the flour, baking powder, salt, and nutmeg. In a large bowl, add the eggs and brown sugar. With a vigorous arm, whisk the eggs and brown sugar together until it lightens and thickens, about 2 minutes. Add the cooled butter to the wet mixture and whisk until combined. Add the flour mixture to the wet ingredients and mix with a spatula until no flour lumps remain.
4. Scrape the batter into the prepared baking pan and spread it into an even layer, smoothing the top. Bake until the surface wrinkles, the edges pull away from the sides of the pan, and the edges are golden brown, about 25-30 minutes. Let cool completely on a baking rack before topping with melted chocolate.

Make the melted chocolate:

1. Place half the chopped chocolate or chocolate chips in a double boiler (a glass or heatproof bowl set over a pan of simmering water). Melt the chocolate until it is smooth and fluid. Remove the bowl from the pan and add the second half of the chocolate, stirring again until smooth.

Top the bars with chocolate:

1. Use a whisk to splatter the melted chocolate over the cooled bars. Sprinkle with additional sea salt.
2. Use the parchment paper to lift the blondie slab out of the pan and transfer it to a cutting board. Cut into 2×2″ squares or your desired size.