Try to replace this line

		$html .= '<button type="submit" class="button alt">' . esc_html( $product->add_to_cart_text() ) . '</button>';

with this one

		$html .= '<button type="submit" value="' . $product->get_id() . '" class="button alt">' . esc_html( $product->add_to_cart_text() ) . '</button>';