For the peanut butter filling, whisk together the peanut butter and confectioner’s sugar until smooth. Set aside.
Preheat the oven to 350F. Line an 8-inch square baking dish with parchment paper, letting the paper go up the sides so you can easily lift the cake out later on.
In a medium bowl, combine the sweet rice flour, sugar, milk, oil, eggs, and vanilla, whisk until smooth. Don’t worry about overworking the batter when making the cake, sweet rice flour is GLUTEN FREE.
Pour half the batter into the prepared baking dish. Used 2 small spoons or small teaspoon ice cream scoop to drop spoonfuls of the filling evenly across the batter, then pour the remaining batter over the filling. Bake, uncovered, for 20 minutes.
While the mochi is baking, place the peanuts in a food processor or blender and pulse until finely chopped. Remove the mochi cake from the oven, sprinkle of the crushed peanuts across the top. The cake maybe puffy so spread nuts as evenly as you can. The cake will fall as it cools.
Return the cake to the oven and bake until the center bounces back when pressed, an additional 15 to 20 minutes. Let cool about 20 minutes before trying to lift the parchment paper to remove the cake from the pan.
Enjoy warm or at room temperature. The mochi slices cleanly when cool, but is delish warm.
Store in an airtight container and keep at room temp if cool, or refrigerate. Microwave pieces on high with 15-second blasts to refresh.