Preheat oven to 350°F. Grease baking pan with butter. You can use any of the following pans: 8” x 8” or 9” x 9” square baking pan,
In a medium bowl, combine ube extract, ube halaya, coconut milk, melted butter, sugar, eggs and vanilla until smooth. In a separate large bowl, whisk glutinous rice flour, baking powder and salt. Pour wet ingredients into dry ingredients. Whisk until you achieve a smooth consistency.
Pour batter into the prepared baking pan. Give the pan a few hard taps on the counter to knock out air bubbles. Bake for about 60 minutes, or until center is set but bounces back when pushed, and edges are brown.
Cool the ube mochi on a wire rack for 1 hour or until the mochi is room temperature. Waiting for it to cool down will give a chewy texture instead of a gooey and custardy inside. Set pan on a wire rack to cool fully to room temperature. If you can’t wait until it cools to room temperature, the mochi cake will be more custardy and gooey but not yet its maximum chewiness.